Based on the Japanese novel All You Need is Kill by Hiroshi Sakurazaka, Edge of Tomorrow tells the story of Major William Cage (Tom Cruise), a man who is forced onto the front lines for a major military operation against invading aliens known as "Mimics." Untrained and unprepared for combat, Cage is killed within minutes - only to wake up 24 hours earlier with no choice but to relive (and die) the same day over and over.
Like many time travel (or time loop) stories, Edge of Tomorrow relies on heady exposition and mind-bending sci-fi ideas which may confuse certain moviegoers. For that reason, we're here to help breakdown the Major Cage's ability to "reset" time, as well as explain the film's ending. Our discussion is going to be full of SPOILERS from here on out - for both Edge of Tomorrow and the All You Need is Kill source material. READ NO FURTHER unless you’re all caught up. You have been warned.
MAJOR SPOILERS FOLLOW
The Source of Time Looping
In the opening moments of the film, we learn that Major Cage is a government-sponsored talking head who refuses to document the UDF campaign "Project Downfall" from the front lines. Branded a deserter, Cage is forced into military service (as part of J-squad) on the eve of a massive offensive, waking up at Heathrow airport in handcuffs (the starting point for the time loops he experiences throughout the film).
In spite of rigorous planning and secrecy, the Mimics see the attack coming and the offensive turns into a massacre - wiping out humanity's last line of defense. On the ground, Cage watches as J-Squad and UDF war hero Rita Vrataski (Emily Blunt) are mercilessly slaughtered by orange-tinted Mimic drones. During the fracas, Cage comes face to face with a blue colored "Alpha" Mimic, exterminating the creature as its acidic blood rains down on his face, killing him. That Alpha blood gives Cage the ability to "loop" (aka reset time by a day).
Through countless trial and error attempts, Cage manages to befriend Vrataski and her physicist confidant Dr. Carter (Noah Taylor) who explain their theories on Mimic physiology. Carter asserts that the Mimics, named for their uncanny ability to adapt to human military strategies, are a highly-evolved hive mind capable of controlling time. At the center of the collective is the "Omega" Mimic, an extremely large and powerful creature that serves as the brain of the Mimic force but is hidden far away from the battlefield for its own protection.
Where the normal orange-colored Mimics are basic workers/pawns, sent into battle for the sole purpose of killing enemies, the Omega also employes the use of the blue-tinted Alphas, who serve as the eyes and ears of the Omega on the front lines. Unlike basic Mimics, Alphas are precious to the collective, and thanks to their direct link to the Omega, contain the head creature's time-controlling essence in their blood. Should an Alpha die, the Omega resets time but retains the knowledge of everything that went wrong on the battlefield - allowing the creature to make tactical adjustments accordingly, and gain an upper hand in the new time cycle.
The highly-evolved power to wind back the clock is responsible for the extraterrestrial's unrelenting adaptation to human military efforts, as well as the reason Mimics were able to see Cage's invasion force coming: because the Omega had already watched it all happen in a previous timeline, and had mapped out a different strategy following one or many resets. In effect, the Omega is able to turn any fight that it experiences into an elaborate trap - until Cage inadvertently hijacks the time-loop ability.
The Rules of Time Looping
At the time of his first death, Cage is drenched in the blood of a dying Mimic, essentially transferring the Omega's time control ability to Cage. After saving her on the French battlefield, Vrataski reveals that she had experienced the same looping phenomenon during an assault at Verdun, but subsequently lost the ability after she was severely injured and bleeding out. During her time looping, she worked with Dr. Carter to understand how the time reset ability actually worked - attempting to find out if it could be transferred to others via physical contact, blood, or other bodily fluids.
The pair came to the conclusion that only one organism (Mimic or Man) could be in charge of the reset at any given time - meaning that when Rita acquired the ability, the Omega no longer had control, giving Vrataski (and later Cage) a temporary edge. The Alpha blood also caused Vrataski (and eventually Cage) to have visions of the Omega's hidden location, and Dr. Carter surmised that if they could find the creature in time, before a reset, they could wipe out the entire Mimic invasion force - since the creatures were all extensions of the Omega. As it turns out, Vrataski and Cage's "visions" were actually traps, planted by the Omega after it had figured out who was in control of the loop - by drawing Rita (and later Cage) to an isolated location, the Omega hoped to reclaim its looper blood, so that the Mimics could regain control of the time reset.
Cage manages to survive the Omega's trap (by drowning himself) and the encounter encourages him to revisit an abandoned piece of technology that Dr. Carter had built - based on the appearance of Vrataski's original visions - to track the Omega by hijacking an Alpha's connection to the Omega. After a number of resets, Cage and Vrataski successfully acquire the gadget, and Cage uses the device on himself (since his body contains Alpha blood), quickly locating the Omega. Yet, during their attempted escape with the device, UDF infantry destroy their getaway car, leaving Cage injured and bleeding out. He awakens hours later with an IV pumping fresh blood into his arm, having lost the ability to reset time - just like Vrataski had months earlier.
As a result, control of the loop reverts to the Omega - but, unless an Alpha is killed, Cage and Vrataski still have time to travel to the creature's location and destroy it. The pair enlist the help of J-Squad, who Cage manages to convince of the time loop (thanks to the countless hours he's spent with each of them in prior loops), and the soldiers set out for Paris - where the Omega is concealed underneath the Louvre. No longer able to reset time, Cage and his team only have one shot to kill the Omega, and are tasked with the added challenge of not killing an Alpha. As mentioned, should the team kill inadvertently kill an Alpha, the Omega would reset time, and Cage would not retain any of the memories of his previous exploits.
The Ending Explained
J-Squad is killed-off one at a time while escorting Cage and Vrataski to the Lourve - where entry to the Omega's lair is guarded by an Alpha. Vrataski sacrifices herself in order to distract (without killing) the Alpha, while Cage tries to destroy the Omega, which is hiding in a flooded portion of the parking garage. As Cage swims downward, he is impaled (and mortally wounded) by the Alpha, who spears him through the chest with a tentacle; in spite of his injury, Cage manages to release a cluster of grenades. The explosion kills the Omega, and since it is the brain of the Mimic collective, its death causes the remainder of its kind on Earth to wither and die, putting an abrupt end to the alien invasion.
Even though Cage survives the detonation, he is doomed to die - that is, until the Omega's blood, rising through the water, seeps into his wound. As Cage dies he regains control of the Mimic's ability to reset time - this time waking up hours before he was ever arrested (his usual start point in the loop). Furthermore, certain actions from the previous loop are retained - while others are not. The Mimics are dead following an unknown event (unknown to everyone but Cage) below the Louvre. Yet, J-Squad and Vrataski have all been resurrected, with no knowledge of their role in eliminating the Mimics during the prior time cycle.