Tom Cruise awakens from a doze on a helicopter trip to London in the UK. He spends a lot of time in front of a portable mirror, looking at his face and dressing his hair. Getting out of the helicopter, many journalists ask him questions about an imminent attack. Tom answer them arrogantly that he has an important meeting with the general and that he will surely come back to them with more info, as he always does. He meets with the Commander of the Western allies General Brigham, debriefing him about an asteroid carrying an alien race called The Mimics crashed into Europe and quickly devastates the continent. The General wants Tom to help him boost his public image or prepare for a post-war political career. Tom sounds very sure of himself, that image is today’s battlefield. Tom goes too far so the General orders Tom to accompany the first wave of Allied troops to land in France and provide ground coverage for the media. Tom, who was a ROTC Cadet in College and has no experience in the field, tries various tactics to avoid the deployment, showing cowardness, culminating with trying to blackmail the General via the media. At first the General seems to relent, but has his guards tazer Tom instead.
Tom 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 Tom had tried to desert and had now been demoted from an Officer to Private. Tom is assigned to a band of eccentric soldiers, J-Squad, who are caught gambling by Farrell when Tom is introduced to them. Farrell forces J-Squad to eat the cards they are gambling with and then instructs them to get Tom ready to deploy with them tomorrow. Tom is immediately taken by the squad for Physical Training (PT).
The next day is invasion day and Tom 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, Tom grabs a claymore mine as a giant blue Mimic jumps on him. The ensuing blast kills the Mimic and Tom, but not before the blue Mimics blood seeps onto Tom.
Tom 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 tries to prevent Vrataski from diing. Vrataski tells Tom “Find me when you wake up” before they both die again.
Tom 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, Tom 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 Tom gets an explanation from Vrataski and Carter: Vrataski was also stuck in a time loop in a previous fight, 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 Tom 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. Tom 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 Tom, killing him a few times when he is injured so that he awakes on the duffels and starts over again. Tom and Vrataski become much improved in combat on the beach, but despite everything, they both keep dying and Tom loops over and over. Tom grows attracted to Vrataski and watching her die over and over again also starts take an emotional toll.
Tom 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 Tom 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, Tom and Vrataski focus their efforts on surviving on the invasion beach and cutting inland to Germany.
After multiple failures and deaths, Tom and Vrataski finally make it inland. During the trip(s) they bond, with Tom 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 Tom is trying to coax her to stay in the farmhouse while he goes alone and confronts Tom. Tom 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, Tom decides not seek out Vrataski and instead escapes the base. However, the Mimics end up attacking London and killing everyone, thus sending Tom back all over again.
Tom 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, Tom, 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 Tom 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 Tom’s leg. Tom then sees the true location of the Omega: in the Louvre Museum in Paris. After seeing this, their getaway car crashes, and the airbag knocks out Vrataski before she can kill Tom.
Tom 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, Tom decides to go back to the base and find the misfit J-Squad to take them to Paris. Toms 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 Louvre, the Mimics crash their airship and in the ensuing firefight half of J-Squad are killed. Tom, Vrataski, and the three remaining J-Squad members decide to use the remaining engine power to slide the airship into the Louvre and detonate grenades to kill the Omega. The rest of J-Squad sacrifice themselves in the slide, and Tom and Vrataski make it into the museum. Once there, Vrataski tells Tom 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 Louvre. Vrataski says neither one of them will survive the ordeal. Tom confess he was a coward before they met, she does not believe it and ask him how he could have changed so drastically? He answers now he has someone to fight for, a goal in his life. They share a kiss.
Tom dives underwater to get to the Omega. After a brief chase, the Alpha catches and kills Vrataski. The Omega senses Tom and sends the Alpha back to catch Tom. The Alpha dives after Tom, but he 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 Tom also dies, the Omegas blood seeps into him, much like the Alphas blood did at the start.
Tom awakes on top of the duffel bags, with the same Sgt. calling him ‘maggot’ and Farrell again taking him to J-squad. Tom is really confused while letting the loop goes. Sgt. Introduce him the J-squad and explain to the soldiers that Tom is here to make interviews about the squad and how they will act if one day the country would be under attack. Tom looks around, reallyzing he is out of the loop. He 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. Tom runs out of the squad looking for Vrataski; he finds her training hard with other soldiers and stares at her lovingly. She notices his starring and comes to him. He tries to escape but she catches him roughly, asking him why he looks at her like that. He just smiles stupidly, while she is getting more upset. He invites him for a drink, mentionning her favorite drink. Intrigued, she follows him while getting even more upset. He whispers “now I have someone to live for”.