After Old Flash realized how many people had died because he was being slowed down by the broken speed force, he quickly went and killed Guerilla Grod for ever messing with the speed force. He then travels back in time to correct it with his first stop being 4 years earlier to stop Mirror Master from accidentally killing 11 people by showing him a newspaper from the future. So, he saves everyone and leaves Mirror Master to die before running 5 years further back in time just to be there for Captain Cold's final moments.
He then moves back four years more where he instantly catches Trickster informing him that he's going to end up offing himself if he doesn't stop crying. And so after multiple stops, Blue Flash gets to the year where the reverse Flash was about to kill Wally West and therefore instantly snapped his neck. Iris and Wall-E were shocked that Flash just killed someone only for their Flash to arrive and begin fighting his future self, causing an explosion that killed him.
Blue Flash nonetheless continued his journey and reached the present day where he takes Current Barry, showing him how the speed force is wounded because of reverse flash's first time travel. He therefore fights him, stating that he needs him as a sacrifice to create an explosion that will seal the wound. Blue Flash manages to trigger the explosion, finally sealing the wound, and somehow starts looking young again, realizing that he's been given a new chance as he quickly goes to his then girlfriend Patty, while the real Flash finds himself far away in the speed force where a madman offers to help him get back to the right timeline.
Blue Flash, however, goes to work to live his life normally, where he gives clues to cases that will be solved in the future. He also gets work done a lot better as he straight up kills guys that will become murderers in the future until Iris once sees him about to kill and stops him. Which only gets worse when Patty finds a body that could have only been killed by a speedster and therefore teams up with Iris to stop him.
Blue Flash, however, didn't care as he later throws Patty away and gets ready to kill a guy that was about to kill hundreds of people. But when the villain blows up his hand with electricity, Barry breaks down, revealing the entire truth about how he isn't the real Barry, stating that he was just trying to fix things but just made them worse. However, after stopping the villain and letting him live, expressing that he isn't that man anymore, the real Flash returns back home, but with a crazy villain.
