The movie opens with a montage of newsreels summarizing an alien invasion of earth. An asteroid carrying an alien race called The Mimics has crashed into Europe and quickly devastates the continent. Just when things look hopeless, an allied army of humans utilizing new mechanized suits wins a major battle in Verdun, and a Special Forces Soldier named Sgt. Rita Vrataski (Emily Blunt) is hailed as a hero after being credited with hundreds of Mimic kills. She becomes an instant icon and a recruiting boon for the human military as the united human armies try to use the momentum for Verdun to launch a multi-pronged final assault on Europe. One of the talking heads on the newsreels selling Vrataski and the war effort is Major William Cage (Tom Cruise) of the US Army.
Cage awakens from a doze on a helicopter trip to London in the UK the morning of the day before the operation to retake Europe. He meets with the Commander of the Western allies General Brigham, assuming the General wants Cage to help him boost his public image or prepare for a post-war political career. Instead, the General orders Cage to accompany the first wave of Allied troops to land in France and provide ground coverage for the media. Cage, who was a ROTC Cadet in College and has no experience in the field, tries various tactics to avoid the deployment, culminating with trying to blackmail the General via the media. At first the General seems to relent, but has his guards tazer Cage instead.
Cage wakes up on a pile of duffel bags in the afternoon at Heathrow Airport, which has been turned into a military base. A Sergeant shoves boots into his hands and calling him 'maggot' before another Sergeant named Farrell takes over. It turns out General Brigham sent orders to Farrell stating Cage had tried to desert and had now been demoted from an Officer to Private. Cage is assigned to a band of eccentric soldiers, J-Squad, who are caught gambling by Farrell when Cage is introduced to them. Farrell forces J-Squad to eat the cards they are gambling with and then instructs them to get Cage ready to deploy with them tomorrow. Cage is immediately taken by the squad for Physical Training (PT).
The next day is invasion day and Cage is a nervous wreck. He is fitted into a suit and the members of J-Squad don't even bother teaching him how to take his weapon off safe mode as they believe he will die quickly. As soon as they are about to drop from aircraft onto the French Coast however, the back of their airship explodes and the squad drops down prematurely. Even though the operation to retake Europe was suppose to be a surprise attack, the Mimics are completely prepared and ambush and slaughter the humans. Sgt. Vrataski also lands on the beach but is killed within minutes. The surviving members of J-Squad are all killed rapidly. Still shell-shocked and about to die, Cage grabs a claymore mine as a giant blue Mimic jumps on him. The ensuing blast kills the Mimic and Cage, but not before the blue Mimics blood seeps onto Cage.
Cage awakes in a panic on top of the duffel bags, with the same Sgt. calling him 'maggot' and Farrell again taking him to J-squad. Despite his desperate attempt to explain what is happening and that the Mimics will ambush everyone, nobody believes him. Each time, he is deployed and dies, only to wake up again on the duffel bags. With each loop, he becomes slightly more adept at trying to save the members of J-Squad and Vrataski, but the Mimics are too numerous and always kill everyone. In one loop, he survives long enough to tell Vrataski that he is stuck in some time loop. Recognizing what happened, Vrataski tells Cage "Find me when you wake up" before they both die again.
Cage awakens on the duffel bags again, but starts cooperating with Sgt. Farrell and J-Squad in order to buy himself time to find Vrataski. After several fatal attempts in the base at slipping away, Cage finally manages to get away from PT and finds Vrataski in a training bay surrounded by training drones. The special forces soldiers are all hostile towards him and Vrataski greets him rudely, but when he explains Vrataski asked for him, she whisks him away to a weapons building bay where her acquaintance Carter works. Once there, they go to a back room and Cage gets an explanation from Vrataski and Carter: Vrataski was also stuck in a time loop in Verdun, explaining how she came to be able to kill hundreds of Mimics single-handedly by the final loop. The loop was created by the blood of the large blue 'Alpha' Mimic (ordinary Mimics are human-sized and orange), but once the looper receives a human blood transfusion, the loop is broken and the person will die permanently. As such, Vrataski instructs Cage to always make sure he dies fully instead of being accidentally resuscitated like she was post-Verdun, an act which broke her out of the loop cycle. Carter was formerly a researcher in General Brigham HQ, but his theory that all Mimics are just neural manifestations of one overmind 'Omega' alien creature led him to create a device that could locate the Omega if inputted into an Alpha led to him being fired. Cage tries to process all of this but Vrataski states that for them to find the Omega first they must survive the invasion tomorrow. She then proceeds to train Cage, killing him a few times when he is injured so that he awakes on the duffels and starts over again. Cage and Vrataski become much improved in combat on the beach, but despite everything, they both keep dying and Cage loops over and over. Cage grows attracted to Vrataski and watching her die over and over again also starts take an emotional toll.
Cage receives a mental vision in one loop that the Omega is in a dam in Germany. Vrataski and Carter believe the visions are due to Cage in effect being an 'Alpha' after being infected with the blue blood, and that the Alphas and the Omega have the power to manipulate time and perception. As such, Cage and Vrataski focus their efforts on surviving on the invasion beach and cutting inland to Germany.
After multiple failures and deaths, Cage and Vrataski finally make it inland. During the trip(s) they bond, with Cage noting down the details of the things Vrataski says on each loop. In one loop, Vrataski mentions having a significant other in Verdun who she had to watch die hundreds of times until she was out of the loop and he was dead for good. After they find a helicopter in a farmhouse, Vrataski realizes Cage is trying to coax her to stay in the farmhouse while he goes alone and confronts Cage. Cage admits they have looped to the farmhouse multiple times already, but every single time Vrataski dies, so he is trying to spare her from dying again by going alone. Shocked and offended, she intentionally gets them killed. In the following loop, Cage decides not seek out Vrataski and instead escapes the base. However, the Mimics end up attacking London and killing everyone, thus sending Cage back all over again.
Cage eventually succeeds in making it to the German dam alone, but finds nothing and is almost imprisoned by the Mimics before he manages to drown himself. In the next loop, Cage, Vrataski, and Carter put the pieces together and figure out the visions, and Vrataski's success in Verdun was all a ruse by the Omega Mimic Overmind; the Omega had let the humans win at Verdun in order to lure the humans into invading Europe, where all the human armies would be ambushed and annihilated. With no other leads, the device Carter made which got him fired is their last shot. Vrataski and Cage switch focus to getting the device, which is in the possession of General Brigham. After several fatal failures, they manage to get the device and stab it into Cage's leg. Cage then sees the true location of the Omega: in the Louve Museum in Paris. After seeing this, their getaway car crashes, and the airbag knocks out Vrataski before she can kill Cage.
Cage awakens strapped to a hospital gurney, under arrest. He realizes he is out of the loop now as he was given a blood transfusion following the car crash. Vrataski breaks the two of them out, but with no more loops left and only several hours before the doomed invasion, Cage decides to go back to the base and find the misfit J-Squad to take them to Paris. Cages now-intimate knowledge of J-Squad and Vrataski credibility convinces J-Squad, and they all take an aircraft and fly to Paris.
Once at the Louve, the Mimics crash their airship and in the ensuing firefight half of J-Squad are killed. Cage, Vrataski, and the three remaining J-Squad members decide to use the remaining engine power to slide the airship into the Louve and detonate grenades to kill the Omega. The rest of J-Squad sacrifice themselves in the slide, and Cage and Vrataski make it into the museum. Once there, Vrataski tells Cafe she will distract a patrolling Alpha Mimic while he will go and detonate a grenade belt on the Omega, which is submerged underwater at the bottom of the Louve. As Vrataski says neither one of them will survive the ordeal, they share a kiss. Vrataski laments that she wishes she had more time to get to know Cage better and takes off.
Cage dives underwater to get to the Omega. After a brief chase, the Alpha catches and kills Vrataski. The Omega senses Cage and sends the Alpha back to catch Cage. The Alpha dives after Cage and stabs him, but Cage pulls all the grenade pins and drops the belt into the Omega. The Omega explodes and dies, and all the Mimics die with it. As Cage dies, the Omegas blood seeps into him, much like the Alphas blood did at the start.
Cage wakes up shock and finds himself on the helicopter trip to London from the morning before the invasion. He lands and sees on the news that after mysterious explosions in Paris, the Mimics have all died off and the human forces will commence sweeping through Europe the next day. All the members of J-Squad are seen alive and well as Cage arrives in the base. Cage goes to find Vrataski; but whereas now everyone treats him with respect as he is a Major once more, when he finds Vrataski she greets him with the same rudeness as she did each time they first met in every loop. Seeing this, Cage breaks out into a big grin.
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