After Barry Allen tried to prevent his mother’s death, he created an alternate future on the verge of destroying itself. At Thomas Wayne’s behest, Flash went back in time to stop himself, and a new version of the universe was created out of the old that merged the distinct and separate DC, Vertigo, and Wildstorm entities.
Until now, Allen believed he was responsible for the New 52 universe, but a sudden appearance by pre-Flashpoint Wally West trying to escape the Speed Force turns everything on its head.
The reveal — you should read DC Universe: Rebirth #1 if you need a primer — is that someone else existing outside of time and place had a hand in creating this new timeline, bending and removing events, relationships, and key factors in order to keep the heroes weakened.
Flash: Rebirth #1 takes readers through what happened before, during, and after Wally West appeared to Barry, starting with a murder case that reminds Allen of his mother’s death — the very thing that kicked off Flashpoint.