Young barrister Alan McKim (John Gregson) returns home from the courts in Genevieve, a 1904 Darracq, as his wife of three years, Wendy (Dinah Sheridan) brings home the shopping and starts to prepare lunch. While she does so, their friend Ambrose Claverhouse (Kenneth More), a brash advertising salesman, arrives in a 1905 Spyker to invite Alan and Wendy to a party that he is having in Brighton the following night for them, himself and a woman named Rosalind Peters (Kay Kendall), a fashion model who became Ambrose's girlfriend after he met her at the races.The next day, Alan and Wendy drive Genevieve to Hyde Park, the starting point of the annual London to Brighton Veteran Car Run. There, they meet Rosalind for the very first time when she arrives in a taxi, bringing her pet St. Bernard for the journey because their porter was too ill to look after him. They escort her and carry her luggage to the Spyker, and are soon met by Ambrose after he returns from the telephone. When it comes time for Ambrose to take his car to the start, the engine backfires when he cranks it and frightens Rosalind's dog, who jumps from the Spyker and runs through the crowd with Ambrose in pursuit. Eventually, one spectator treads on the dragging leash allowing Ambrose to catch the dog. They return to the Spyker and prepare to start the run to Brighton. After the starter drops the flag, the car jerks forward causing the dog to growl. Ambrose half-turns and raises his arm to fend off a possible attack, and the car swerves violently straight toward the starter, forcing him to leap back in surprise. The Spyker then catches the rope fence on the edge of the enclosure, drags it for about twenty yards and pulls a couple of fence posts out before it charges onto the road. Ambrose becomes a little more relaxed after the Spyker passes Buckingham Palace, though he gets annoyed again after its narrow wheels get caught in the tram lines beyond Westminster Bridge.Alan and Wendy's start to the Brighton Run is far less spectacular than that of Ambrose and Rosalind, but after they leave suburban London behind for the countryside, Alan remarks on how Genevieve has been behaving herself, moments before she coughs, wheezes and passes out. Wendy decides to pour some coffee while Alan gets his tools to fix Genevieve up. He cranks the starting handle unaware that Genevieve is still in gear, and her sudden lurch causes Wendy to spill some coffee on her dress. She goes behind a nearby hedge to change her clothes before they set off again, passing a signpost indicating that they are 23 miles from London. Meanwhile, Ambrose makes so much good time in his Spyker that he decides to surprise Rosalind by turning off the main road into a narrow drive that leads to a country club where they have lunch.With eighteen miles to go until they reach Brighton, Alan gets the idea of stopping off at a nearby pub for lunch for him and Wendy when Genevieve gets quite noisy inside, forcing him to stop again. Wendy has a surprise of her own for Alan, pulling out a picnic basket she had secretly packed the night before in case they needed it. She finds a place where they can spread their food, but Alan would rather eat while he continues working on Genevieve. Their idyll is broken by Ambrose when his Spyker pulls up beside the stricken Genevieve and he makes a few remarks at Alan's misfortune. After Ambrose moves away, Alan closes Genevieve's bonnet and turns the handle, and her engine breaks into a fine rhythmic beat. As it does so, Alan and Wendy hear a bang and see a cloud of black smoke momentarily envelop the Spyker a few hundred yards down the road. They drive up to the Spyker and Alan repeats many of the lines that Ambrose had used moments before when Genevieve was the one in trouble. Ambrose begins to move in a menacing way towards Genevieve, but Alan lets out the clutch and drives her to safety.Further on, something else in Genevieve's inside snaps and causes the steering wheel to break loose in Alan's hands. After Genevieve veers wildly across the road, Alan finally brings her to a stop and examines beneath her bonnet. When he hears the horn of Ambrose's car, Alan runs back down the road waving his arms, but Ambrose drives straight toward Alan and forces him to leap into the verge. The other veteran cars begin to arrive in Brighton at the end of the Run, and Ambrose and Rosalind surprise the other drivers by arriving and stopping by the time-keeper. After Alan bends his knees a few times, he begins to fit back the engine parts and on the verge of dusk, he gets Genevieve started again, waking a sleeping Wendy. Alan realises that for the first time in the 25 years that he and Genevieve have done the Brighton Run, they will have missed the dinner.In darkness, Alan, Wendy and Genevieve arrive at their usual plush Brighton hotel, but as a porter comes to welcome them, Alan tells Wendy to wait in the car while he goes into the hotel lobby. When he returns a few minutes later, he explains that he cancelled their room booking the day before after Wendy had told him she wasn't coming (at the previous day's lunch, Alan heard Wendy say to Ambrose that she couldn't possibly tell Alan her true feelings about the London to Brighton Run which she confided in Ambrose, only for him to tell Alan that she thought that it was a bore), but the lobby staff gave him the name of another place that sounded wonderful.Moments later, they arrive at the Grand Palace Marine Hotel and discover that it is run-down and dingy. When they check-in, the hotel's proprietress (a cameo by Joyce Grenfell) tells them that their room is one that they don't normally let unless the other rooms are taken because it is rather noisy, and the permanent resident who had previously occupied the room until she died a week before was a totally deaf lady. She goes on to say that the room is on the fifth floor but the hotel doesn't have a lift and the hall porter is having supper, so Alan tells her that he thinks he can manage carrying their luggage themselves. When Wendy enquires about a bath, she finds out that hot water is only available between 2:30pm and 6:00pm. They eventually arrive at their room and find that its small windows only offer a view of a nearby brick tower, the purpose of which soon becomes obvious when they hear a metallic clanking sound, followed by the resounding and shattering sound of a clock bell striking ten, which causes Wendy to laugh.Finally, Alan and Wendy join Ambrose and Rosalind at a nightclub for the after-dinner drinks party for four that Ambrose had planned. After Rosalind gets drunk with champagne, she decides that she wants to play the trumpet with the club's orchestra. She performs a hot jazz solo that ends with a high note that she has no trouble with hitting (mimed to a rendition by Kenny Baker), then moments later she falls fast asleep to Wendy's amusement.Wendy keeps laughing as she and Alan climb the steps to their room, despite a "shush!" of protest from a permanent resident, but the laughing stops when Alan and Wendy argue about Ambrose's supposedly romantic attention towards Wendy. At one point when Alan asks Wendy loudly about the 1949 Run, three sharp taps on the wall from the adjacent room spoil his attitude and cause Wendy to begin giggling again. When Wendy wants Alan to make love to her, he thinks she isn't taking the matter seriously, pulls out some old clothes, slams the door shut and goes to the garage to work on Genevieve. The resident in the adjacent room repeats their earlier taps in response to the door slamming, and when Wendy's use of the metalled heel of her shoe to tap back a reply leads to an even louder repetition of the resident's taps, she blows into a paper bag that she grabs from the picnic basket before crashing it against the wall with a sharp explosion, and smiles wickedly after the resident suddenly gives an alarming scream. Before she can contemplate her next move, the nearby tower makes a clanking sound before its clock strikes the next hour.When Ambrose goes for a midnight stroll along the beachfront, he passes the Grand Palace Marine Hotel, then sees a light from a nearby garage that leads him to Alan as he continues tinkering with Genevieve. They exchange some angry words about the abilities of their cars to take them and their girls back to London the following day, which leads Alan to impulsively bet Ambrose £100 that Genevieve can beat the Spyker back to London in a race, despite the Club forbidding that sort of activity. They agree to meet at the Black Fox at noon. Later, when Wendy wakes up to find that she is alone in her bed, she descends the stairs, finds the door of the garage on the other side of the street open and confronts Alan there. He tells her that Ambrose was there the night before and about the bet of a race back to London. Alan follows Wendy back to their room, where he is so tired that he reels and crashes into the bathroom wall, and Wendy helps Alan into bed.When Ambrose and Rosalind arrive at the old country pub that is the Black Fox, he comes out of the bar with a pint of beer and a tumbler of Alka-Seltzer to help Rosalind cure her hangover. Ambrose promises Rosalind a trip to Le Touquet for just the two of them if he wins the bet, then they hear the sound of Genevieve arriving at the pub. Ambrose offers Alan a drink or two, but Alan is keen to start the race, and is surprised when Ambrose sends him on his way without joining him. Ambrose buys another drink, but when Rosalind's lack of interest makes it clear to Ambrose that he only has himself to impress, he begins to wonder how many miles Genevieve has done, and eventually quaffs the rest of his drink. Rosalind's St Bernard initially doesn't want to get into the car, but the yap of a Pomeranian quickly makes him change his mind. Ambrose tries to turn the starter's handle, but can't get the Spyker started. He eventually gets four drinking cyclists to push the car, but still can't get the engine to catch. The pushers drop out with discouragement until one lady remains. Alan continues to drive Genevieve flat out towards London until two motorcycle policemen pull him over for doing 45 miles an hour in a restricted zone. Wendy uses her charms to convince the police not to issue them with a ticket. Meanwhile, the last cyclist manages to push Ambrose's Spyker into action. He drives off with a word of congratulations to her, unaware that she has fallen flat into the middle of the road.Both Ambrose and Alan experience some setbacks on their journeys to London. Ambrose tries to take a shortcut to get around a flock of sheep, only for the Spyker to get bogged at a ford. Rosalind pushes the Spyker until its wheels find firmer ground, but nearly falls into the water, and when they re-join the main road, they find they have lost enough time for the sheep to pass the junction first. Wendy also gets wet after the tin she uses to retrieve some water after Genevieve's engine runs a temperature turns out to be full of holes. Then a young man tells Ambrose that he needs to get the District Nurse because his wife is about to have a baby, and directs him to turn down a narrow side-road. One mile down the track, they reach the District Nurse's cottage, but the only way for Ambrose to get back to the road is to reverse. The District Nurse's car catches up to the Spyker, and soon after Ambrose reverses into a hedge. Genevieve begins to develop a rattle, forcing Alan to ask J.C. Callaghan (Reginald Beckwith) to tow them with his new car. Alan and Wendy argue about whether or not being towed is considered cheating, but their argument stops as suddenly as J.C. Callaghan's car when it reaches the garage, which gets rear-ended by Genevieve when Alan and Wendy are too busy arguing to notice.As J.C. Callaghan tells them that the new car belongs to his wife, Alan hears the sound of another veteran car, and is astonished to find that it is Ambrose's Spyker. Ambrose picks up the tow rope and asks Alan if he has conceded the race. J.C. Callaghan pushes him aside as he, Alan and Ambrose get into a three-way argument in which the two veteran car drivers turn on J.C. Callaghan, leading both Wendy and Rosalind to sympathise with the dapper little man. When Ambrose gets back to asking Alan if he has conceded the race and suggests that he has cheated, Rosalind intervenes by comparing Genevieve's pulling to the Spyker being pushed half a mile by the District Nurse's car. When Ambrose turns on Rosalind, Wendy defends the lady and tells Alan and Ambrose to either call the race off or stop behaving like lunatics. Ambrose tells Alan that he's happy to make it a race in which anything goes, but Rosalind, Wendy and Alan are so full of their own problems that they fail to notice Ambrose stop by Genevieve before he and Rosalind drive away.As they continue driving, Ambrose becomes more like his usual hearty self, and he drives a mile or two off the main road to a secluded glade where he surprises Rosalind with a picnic, something she had wanted the day before. Ambrose gives her a chance to stretch out and relax, telling her that Alan will be hours at the garage and producing a piece of metal that he took from the inside of Genevieve. Rosalind turns on Ambrose by telling him it's a rotten thing to do, and that she has had enough of the picnic.Half an hour later, the garage mechanic and Alan put away the spanners and get Genevieve ready for the road, but after the mechanic tells Alan to start her up, there is no spark of life when he turns the starter handle. They open Genevieve's bonnet in search of something wrong and don't take long to find a gap where a piece of metal should be, in a place from where it could only have been unscrewed, leading an indignant Alan and Wendy to realise they have been tricked. Another hour passes before the mechanic has a replacement part ready and a moment or two later, Genevieve comes back to life as it turns from the garage onto the road back to London. When Wendy tells Alan to forget about the race, Alan has an idea that leads him to suddenly stop the car and cross the road before disappearing into a telephone box.The journey back to London continues with Alan and Ambrose playing more tricks on each other. The landlord of a wayside inn tell Ambrose and Wendy about an accident involving a young couple named McKim, and when they drive back towards Brighton with Ambrose in tears, they see Genevieve going the other way, then Ambrose reports Genevieve as a stolen car, though it doesn't take Alan long to convince two policemen, the ones who had pulled him over earlier for speeding, that Ambrose had made a false report. They wave Genevieve on and soon pull over the Spyker and give Ambrose the third degree. Ambrose becomes alarmed when they suggest that he sign a statement at the station, and convinces the policemen to let him go in order to give him a chance. They start to warn him about what they will do if he plays another trick, but he is back on the road before they get a chance to finish their admonition.By this time, Genevieve has reached West Drayton, on the outskirts of London. After Wendy makes a toilet stop at a dingy pub, Ambrose also stops there and confronts Alan about his disgusting trick he played, leading Alan to mention Ambrose's own tricks. When Ambrose tells Alan that he let him think he had been killed, Alan quotes Mark Twain when he replies that the report of his death was greatly exaggerated. Ambrose answers that he was actually thinking about Wendy, and it's only that thought that is stopping Ambrose from giving Alan the lesson he deserves. Meanwhile at the pub, Wendy and Rosalind exchange thoughts about their men. When Ambrose warns Alan that he is going to lose his temper, a spectator from the crowd that surround them encourages Ambrose to do so, but just as it seems that they are about to come to blows, two policemen hear the encouragement of the onlookers and push their way through the crowd before stopping dead when they recognise the two men at the centre of the trouble. One policeman separates Alan and Ambrose while the other moves the onlookers on to stop them causing an obstruction. When all is quiet again, the policemen warn Alan and Ambrose that if they cause any more trouble, they will press so many charges that they will not be out in time for the following year's Brighton Run. Wendy then admonishes Alan and Ambrose, being unable to believe that either of them could be "hawling like brooligans". Alan and Ambrose are initially startled by her Spoonerism, then after staring at each other, they both begin to laugh. Wendy and Rosalind suggest that the four of them should go home, leading Ambrose to suggest that he and Alan should call the whole thing off and have a party, an idea to which Alan agrees.When Ambrose sees an opportunistic ice-cream vendor, he orders four double cones for them to eat while they wait ten minutes for the pub to open. As they eat, Alan asks Ambrose how they got behind him, and Ambrose tells him about picking up the expectant father. After Alan remarks that they have made jolly good time all things considered, he and Ambrose exchange words that lead Ambrose to hold Alan to the bet, to which Alan expresses the view that as far as he is concerned it is still on, much to Wendy's chagrin. Before they set off on the nine miles to Westminster Bridge, Alan asks Ambrose about how much he thinks Genevieve is worth, before changing the stake - if Ambrose wins, then instead of £100, Alan will give him Genevieve, a turn of events that Wendy doesn't like at all.The two cars line up side by side at a set of traffic lights on the other side of a bridge near the pub. Wendy notices Ambrose slowly creep forward, but he turns to argue at the very moment that the lights turn green. Genevieve gets away as the faster of the two veteran cars, but it doesn't take long for Ambrose to catch up. For a while, the two cars race nose to tail through London's southern suburbs. After emerging out of High Street, both drivers quickly jump on the brakes when they see two children on a pedestrian crossing eating ice creams as they cross leisurely. As soon as they move clear of the Spyker, Ambrose roars away and quickly gains a substantial lead. When Alan tells the children to hurry, the girl turns to look at him, as a result of which her ice falls out of the cone and onto the road right in front of Genevieve's front wheels. She stops to pick it up until her brother drags her, protesting, to the pavement, allowing Genevieve to move on again. At first, there is no sign of Ambrose, but they see him when they come to a long uphill section. Genevieve passes the Spyker just before the crest, but it re-takes the lead on the subsequent descent.The cars come into a narrow winding section of road where they follow a small car driven by an elderly lady at about ten miles per hour. Ambrose realises that the only way for him to retain his lead is to pass her, but a steady stream of oncoming traffic and the narrow road combine to make it difficult. Traffic piles up behind her as she appears to be oblivious to Ambrose's attempts to attract her attention or to pass her, with only a lorry separating the two veteran cars. Ambrose is so busy hooting and shouting at the lady that by the time he realises that she is going to stop completely, he has just enough time to slam on the brakes and stop only inches behind. By the time Ambrose gets his breath back, he realises that the elderly lady had disappeared into a shop. The lorry behind him pulls up immediately behind, completely trapping the Spyker and thwarting Ambrose's attempt to reverse. At that moment, there is a gap in the oncoming traffic large enough to allow Alan to draw Genevieve out into the road to shoot past all three stationary cars.As they get closer to central London, Alan and Wendy have to wait several minutes at a crossroads patrolled by a policeman on point duty, allowing Ambrose and Rosalind to pull up alongside Genevieve. As they do so, an elderly man notices the Darracq and begins a conversation with Alan by remembering that a Darracq was the first car he ever owned. As he keeps speaking, the policeman lowers his hand. Alan tries to end the conversation, but the old man is so taken with Genevieve that he refuses to hear Alan and continues speaking to him. The policeman turns his back and raises his hand again while another line of traffic begins to move. The old man reminisces about how he proposed to his wife in a Darracq like Genevieve at Pangbourne in 1904, and that he thought it was the car that made her say yes. Eventually, he senses that he is holding Alan up, but Alan is now happy to continue the conversation. In the end, he offers to come to the old man's place and let him and his wife ride with them, even giving the man an opportunity to drive if he wants. They finally say goodbye, and when Alan drives off, he is too absorbed in his own thoughts to notice that Wendy is crying until she sobs after Alan tries to share his thoughts with her because she thinks that they have lost Genevieve, and knows how much she means to Alan.Wendy convinces Alan to give Ambrose the money instead of Genevieve, but then they see Ambrose stopped by the side of the road against a wrecked barrow, with fruit and vegetables strewn over many yards, making it clear that Ambrose had had an accident with the barrow. When he hears Genevieve approaching, his interest in the two barrow boys wanes and he hands them several notes to compensate them for the damage, determined to be on his way again. He succeeds in getting away and swings toward the centre of the road to stop Alan passing him in spite of Genevieve's momentum. They are now only 200 yards away from Westminster Bridge. Seeing no traffic oncoming, Alan makes another attempt to pass and Ambrose blocks him again, but just when he seems sure of victory, the Spyker fails to make the final turn onto the bridge's approach. As Alan turns toward the bridge and sees Ambrose wrestling with the Spyker's steering wheel, he realises what happened - the narrow tyres of the Spyker were caught in the tramlines again. Alan's moment of victory appears to be short-lived, for only yards away from Westminster Bridge, one of Genevieve's tyres burts. Alan pulls over and applies the brake to look at it, but before Ambrose can recover, Genevieve's brakes fail and the Darracq rolls far enough forward to reach the bridge, ensuring that Alan and Wendy are the winners of the bet - and of Genevieve.