In the one-shot story we are introduced to Bruce Wayne of earth -52.
Bruce is after Barry Allen a.k.a. the Flash. Bruce is convinced that someone else should have the speed force. Prior to their feud, Bruce’d asked Barry to go back in time and erase some horrible events that had happened. Barry refutes the idea of going back in time due to his own regretful experience with time travel.
After a heavy clash between Batman and Flash, it seems that Flash has the upper hand by removing Batman from his vehicle. However, Barry was caught off guard, which gave Bruce enough time to inject him with the cryostasis formula. Despite Flash’s increased metabolism, he managed to knock him out within three seconds.
Bruce has only one problem left: how will he gain Barry’s powers?
There’s one method which is to steal it from him. He chained him in front of his vehicle and drove as fast as possible, repurposed the engine of the cosmic threadmill thus allowing him to harness Barry’s connection to the speed force. Barry responds that this is a ridiculous idea, because it will rip them both apart by doing so.
Bruce is now the Flash, but there’s a catch. He’s “fused” with Barry, or at least, he can hear his voice in his head.
Ultimately, Bruce has still realised his dreams as he’s now the most fearsome, but more importantly, fastest man alive!