Newly married Mary Sage (Shirley Eaton) is distraught when her husband Charlie (Bob Monkhouse) receives his call-up papers during their wedding breakfast. Both Charlie and his father forgot to file for a 28-day extension with the Government on account of Charlie's wedding. He travels to Heathercrest National Service Depot, meeting fellow recruit Horace Strong (Kenneth Connor), a terminal hypochondriac who is devastated at having been passed as fit. Horace jumps at the sound of everything.The new recruits are assigned to Sergeant Grimshawe (William Hartnell). Grimshawe is retiring from the army in the next 10 weeks and takes on a £50 bet with Sergeant O'Brien (Terry Scott) that his last bunch of squad enlists will be his first Champion Platoon. (Terry Scott was paid £50 for his part in the film). Grimshawe has never had a Champion Platoon in his army career and wants to retire on a high.Grimshawe knows that his squad will take some work to put them into shape. 50 pounds are riding on the outcome and Grimshawe does not want to lose. With beady-eyed inspection from Captain Potts (Eric Barker) and disgruntled support from Corporal Copping (Bill Owen), Grimshawe decides to use some psychology and treat his charges kindly rather than simply shouting at them.But basic training doesn't start well, and he struggles to take his squad through it. They include failure Herbert Brown (Norman Rossington), upper-class cad Miles Heywood (Terence Longdon), rock 'n' roller Andy Galloway (Gerald Campion), delicate flower Peter Golightly (Charles Hawtrey) and supercilious university graduate James Bailey (Kenneth Williams). Grimshawe's attempts seem doomed. Brown has been through three basic training courses and has managed to flunk each one of them. Even the army considers him good for nothing and has excused him from doing anything dangerous on the base, where he can hurt himself or others.Horace is determined to be sick the entire time, but is reluctant to visit the camp doctor, who is a woman Captain Clark (Hattie Jacques). The platoon can't even do attention and at rest commands properly. Golightly is hard of hearing. They fail a kit inspection when Charlie spews fire retardant all over the Captain and the Sergeant.Mary is determined to spend her wedding night with her husband and smuggles herself into the depot to get a job in the NAAFI. Mary meets Norah (Dora Bryan), who is a helper in the NAAFI kitchen. Norah knows that Mary doesn't belong in the kitchen but decides to help her. Charlie believes that he heard Mary on the base, but none of his mates believe him. Charlie finally meets her at the officer's bar. Mary tries to meet Charlie in a room arranged by Norah, but finds Grimshawe there instead.Charlie is eventually able to legitimize this situation when he is able get leave from Potts on compassionate grounds. Strong spends most of his time complaining to the Medical Officer, Captain Clark (Hattie Jacques). Strong has reported sick every day for ten weeks. Clark gets him examined by medical experts, who also say that there is nothing wrong with him.The platoon is in terrible physical shape and can't even do rope climbs. Bayonet practice is a disaster (half the platoon misses the dummy; the other half get entangled in it) and so is slow march practice (they are not in sync and keep kicking each other). Week on week the scores only drop as target practice and ditch swinging all go badly.It is only the adoration of doe-eyed NAAFI girl Norah (Dora Bryan), which he initially rejects, that makes Horace realize his potential and inspires him to become a real soldier. Bailey helps Brown to learn the concepts of basic training and gets him over his failure to learn new things. Heywood comes from an illustrious military family and his ancestors have served as Generals, Admirals, and other high ranks. Potts offers Haywood the chance to move to the officer training school, but Heywood says that he is not a leader of men and decides to stay with his platoon. Grimshawe is proud.The platoon makes an absolute hash of the obstacle course. Potts is sure that the platoon will finish at the bottom of all the platoons that have ever passed the academy in history. The platoon knows that Grimshawe has treated him well. They want to give him a retirement present.On the eve of the final tests, Grimshawe is in despair, but he is overheard bemoaning his lot to Copping. The squad decide to win the best platoon prize at all costs. On the day, they indeed beat the other platoons at all tasks and Grimshawe is awarded the cup for best platoon.