As commanded, Cage hits the beaches the following morning, but things don’t go as might be predicted. To be sure, the battle is disastrous for Cage (and for his fellow Earthlings), but before he’s killed, he experiences a bout of beginners luck and kills an “Alpha” alien, a rare member of the species with the ability to manipulate time.
The Alpha’s blood splashes over Cage’s face and body, absorbing into his system and granting him the ability – albeit inadvertently – to “reset” the day. He thus goes directly from death on the battlefield to waking up the previous day, again in handcuffs, again in Farell’s unforgiving custody, with every event playing out exactly as it did the first time around.
And so it will go for Cage for the bulk of the film: death on the battle field; reset; death on the battlefield; reset. With each death and reset, Cage works to learn something new to help him and his fellow Earthlings defeat the alien invaders, who are on the verge of conquering humanity thanks to their ability to manipulate and reset time and thus anticipate the future.
Only one other person on Earth understands what’s happening to Cage: Sergeant Rita Vrataski (Emily Blunt), who, at one point, also inadvertently acquired Cage’s curse of reliving the same day over and over. Cage seeks her out during one of his resets, leading to a continual cycle where he goes to her every new reset, re-introduces himself, catches her up on the previous loops and works with her to figure out how to defeat the alien invaders based on what’s been learned during previous resets.
That setup might sound tedious, but Director Doug Liman and Screenwriter Christopher McQuarrie do an admirable job of preventing things from bogging down and becoming overly repetitive.