Vandal Savage arrives in Central City and sets his sights on Kendra Saunders. After Vandal attacks Kendra and Cisco, they turn to Barry for help. Realizing how dangerous Vandal is, Barry takes Kendra to Star City and asks Oliver and team to hide her until he can figure out how to stop Vandal. However, things quickly go from bad to worse when a man with wings AKA Hawkman shows up and flies off with Kendra. Meanwhile, Harrison develops a serum to make Barry run faster and asks Jay to test it out.

The crossover continues on Arrow S04E08 Legends of Yesterday (II).

Barry is shocked when Eobard Thawne returns in the most unexpected way, and with a tie to a loved one. Damien Darhk offers advice to Barry but there is a catch. An epic battle begins with Reverse Flash pitted against The Flash, Team Flash, Batwoman, Sentinel and Ryan Choi.

An opportunity presents itself for The Flash to end his lifelong battle with Reverse Flash for good, but the payoff could be too much for Barry and team to handle. Meanwhile, Mia Queen drops in from the future looking to save a lost loved one, and she won't let anything stand in her way.

With The Flash still reeling from everything that just happened, the rest of the team rallies to support him when he needs it the most.