In 2015, an alien race called Mimics arrive in Germany to storm and conquer continental Europe. In 2020, the United Defense Force (UDF) achieves one solid victory over the Mimics at Verdun. In Britain, General Brigham of the UDF commands Major William Cage to cover a massive assault on France. Cage, who has no combat experience, objects and threatens to put the blame on Brigham should the invasion result in failure. Brigham has Cage arrested for disobeying orders and orders him sent to Heathrow Airport, now a military base. Cage wakes up to find Brigham has falsified records to make it appear he is only a private and has assigned him to the misfit J squad under the command of Master Sergeant Farell.
The invasion of France is a colossal failure for the UDF. During the battle, Cage uses a Claymore mine to kill an unusually large blue Mimic but dies when the alien’s blood dissolves him. Cage is jolted awake and finds himself back at Heathrow experiencing the events of the previous morning over again. Cage attempts to warn Farell that the invasion will fail but is not believed. He repeats the loop of dying on the beach and waking at Heathrow over and over again, becoming more skilled in combat each time. During one time loop Cage unsuccessfully attempts to save the life of Sergeant Rita Vrataski, who realises that Cage can loop time. She orders him to locate her the next time he wakes up.
Cage reawakens and locates Vrataski, who takes him to Dr. Carter. Carter is an expert in Mimic biology and explains that Mimics are a superorganism in which the “Omega” controls the cerebrum, while the “Alphas” behave as the ganglia in which the Omega controls ordinary Mimics. If an Alpha is terminated, the Omega resets the day and rearranges its tactics until the battle is won. Cage inadvertently “hijacked” their ability to reset time through his exposure to an Alpha’s blood. Vrataski had this ability at Verdun, using it to win the battle before losing its power when she was wounded in battle and received a blood transfusion. She tells Cage to locate and kill the Omega in order to end the alien invasion and save the human race.
Over many more loops Vrataski trains Cage to excel in combat. After one frustrating lesson Cage visits London and discovers that the Mimics will attack there after defeating the UDF in France. Cage also begins having visions of the dam in Germany that the Omega is hiding in. Cage and Vrataski spend many loops figuring out how to escape the invasion and reach the dam. Cage comes to know Vrataski and falls in love with her, but Vrataski is only interested in the mission. Upon reaching a point where she is killed whatever they do, Cage decides to fly to the dam on his own. Inside the dam, instead of finding the Omega, he is ambushed by an Alpha and Mimic who attempt to strip him of his ability to reset time. Cage preserves the ability by deliberately drowning himself.
Cage and Vrataski infiltrate the MoD to obtain a prototype device built by Carter that can locate the Omega. Cage compels General Brigham to give him the device, and during the ensuing car chase discovers that the Omega is under the Louvre Pyramid in Paris. Cage is captured and seriously injured, he wakes up in a hospital to find he has been given a blood transfusion and has lost the ability to loop time.
Vrataski frees Cage and they recruit J squad to help destroy the Omega before the invasion begins. They fly to Paris, where the other soldiers sacrifice themselves so that Cage and Vrataski can get into the Louvre. Just before luring away an Alpha standing between them and the submerged Omega, Vrataski kisses Cage to thank him for getting her as far as he did. The Alpha kills Vrataski and mortally wounds Cage, but he manages to drop a belt of grenades that kill the Omega.
The dying Cage floats down through the water, into the Omega’s blood. He awakens en route to his first meeting with Brigham, who is announcing on TV that Mimic activity has ceased following a mysterious energy surge in Paris. Cage, still a major, goes to Heathrow and finds Vrataski. She greets him as icily as she did in previous loops, causing Cage to chuckle.