A US passenger airliner in 1989 is about to be struck from above by another airliner on a landing approach. The pilot handles the plane as well as he can while the flight engineer goes back to check on the passenger cabin. He comes back in the cockpit screaming, "They're dead! All of them! They're burned up!"Bill Smith is a National Transportation Safety Board investigator hired to determine whether the collision and subsequent crash of both aircraft was due to some mechanical fault or human error on the part of either pilot. He and his team of investigators are confused by the flight engineer's words on the cockpit voice recorder, as there is no evidence of a fire on board before the plane hit the ground. At the same time, a theoretical physicist named Dr Arnold Mayer has a professional curiosity about the crash, which borders on science fiction. While giving a lecture, he talks about time travel and the possibility of visitors from the future.Time travellers are visiting the present day and stealing passengers from doomed aircraft. In the future, because of pollution, the human population is no longer able to reproduce, so teams are sent in to the past to abduct groups of people who are about to die and keep them in stasis until they will be sent into the far future to repopulate the Earth. While many people in the future are in poor health, some are healthy enough to successfully pass for 20th Century humans, and are given the best of food and care so that they can successfully infiltrate, using reproductions of contemporary clothing.Every incursion into the past causes an accompanying "timequake" whose magnitude is proportional to the effects of the incursion. Each "timequake" causes physical damage in the time from which the incursion has been made. This is why they are abducting people who will not be able to affect the future any further and replacing them with copies of those who would have died. Thus, the flight engineer's strange comment came because all the passengers had been replaced with pre-burned duplicates in preparation for the impending crash.While on a mission to 1963, a time travel operative on board a plane is shot before it crashes, losing a stun weapon as a result. This weapon winds up in the possession of Dr Mayer, setting him on the path to working out what is happening. Twenty-five years later Smith finds a similar artefact among the wreckage of the crash portrayed at the beginning of the film.Worried that the 20th century discoveries made by Smith and Mayer might change history, Louise Baltimore travels back to 1989 to distract Bill Smith and discourage him from pursuing his investigation further. Louise gains Bill's trust and seduces him into a one-night stand, which she hopes will complete the distraction. It is later revealed that Louise becomes pregnant as a result of this encounter. As a consequence of further errors on the part of the time travel team and paradoxical events, Bill becomes even more suspicious. He soon pays a visit to Dr Mayer. Louise materializes from the future and reveals her mission to both of them in the hope that they will voluntarily keep quiet. In a mishap with the stun weapon, Mayer inadvertently kills himself.Mayer was instrumental in the development of the Gate technology that made time travel possible, so his death results in an unresolvable paradox - a force infinity timequake - which will destroy the entire civilisation of the "present" future. The only course of action is to send all of the people who have been collected into the distant future before the Gate is permanently destroyed. Louise goes with Bill to the future.Bill and Louise step through the Gate and disappear: they are transported to another time and place, saving their lives, and allowing them to fulfil a destiny to repopulate the Earth. As a cataclysmic explosion destroys the Gate and the blast wave engulfs him, Sherman the Robot quotes Winston Churchill: "This is not the end. This is not the beginning of the end. It is the end of the beginning."