A visitor from the future tells best friends Bill and Ted that one of their songs can save life as they know it and bring harmony to the universe.

Worlds collide when Barry uses his superpowers to travel back in time in order to change the events of the past. But when his attempt to save his family inadvertently alters the future, Barry becomes trapped in a reality in which General Zod has returned, threatening annihilation, and there are no Superheroes to turn to. That is, unless Barry can coax a very different Batman out of retirement and rescue an imprisoned Kryptonian, albeit not the one...