One of my kids told me about this one. A deliciously dark alternate history story that starts with a good deed. Events ripple out from that change up and down the timestream and the Flash ends up in a rather nasty universe. One of the best parts was the effect on the Wayne family. In this timeline, it’s Bruce who is killed, not his parents. Thomas Wayne becomes a very dark and deadly Batman and the shock turns Martha Wayne into the Joker, his arch nemesis. The Flash works to find the cause of the timeline change and set things right. It’s hard road, but the Speedster is ready to run it.