Back in an alternate 2015, an alien race known as the Mimics invade Earth, guns a’blazin’. They take the world by storm, with Europe being a pretty big focus. Humanity evolves its warfighting capabilities, and somehow in the alternate 2020, Major William Cage (Tom Cruise) is seen as a good candidate to be embedded on the front lines. Stuff happens, Cage gets demoted for desertion and cowardice, and he accidentally gets himself stuck in a time loop. On the plus side, he does get to keep running into Sergeant Major Rita Vrataski (Emily Blunt,) the Angel of Verdun.
Article source: https://www.cinemablend.com/news/2557657/how-edge-of-tomorrows-time-loop-works