After a bomb explodes downtown, the army rolls in and takes over. When Joe learns Iris is writing about 'The Streak' and tries unsuccessfully to have the story stopped, Barry decides to have The Flash pay her a visit.

While Barry tries to find Eddie, a mysterious mind-controlled thief launches attacks on Central City's gold reserves. The team soon discovers that a telepathic gorilla is behind the robberies... and is following Harrison's commands. Meanwhile, Iris confronts Barry, while Harrison prepares to return home.

Wells presents Barry with a life-changing choice. Dr. Martin Stein and Ronnie Raymond return to help the S.T.A.R. Labs team with this final fight.

Grodd returns to Central City and kidnaps Caitlin. Barry and team race to find her before it’s too late. Meanwhile, Cisco plans his first date with the new barista at Jitters, Kendra Saunders and Patty begins to suspect Barry is hiding something from her.

When aliens called the Dominators attack Central City, Barry speeds over to Star City to ask the Green Arrow for help in stopping them. Realizing the battle against the Dominators is bigger than they thought, they track down the Legends of Tomorrow, and Barry and Cisco create a breach to bring back a secret weapon – Supergirl. Once all of the super friends are assembled, they come up with a plan to attack but a shocking secret related to Flashpoint is revealed and suddenly no one is sure who they can trust. Meanwhile, Wally wants to join the battle against the aliens but Iris tells him he’s not ready.

The crossover starts on Supergirl S02E08 Medusa (I). It continues on Arrow S05E08 Invasion! (III) and DC's Legends of Tomorrow S02E07 Invasion! (IV).

When Jesse Quick informs the team that her father has been abducted, Barry, Caitlin, Cisco and Julian voyage to Earth-2 on a rescue mission to save Harry from Gorilla City. As they trek through the forest, Barry and the team are immediately captured and brought to Grodd. Grodd tells them he needs their help to stop Solovar, the leader of Gorilla City, as Solovar wants to invade Earth-1. Meanwhile, back on Earth-1, Jesse and Kid Flash hit the streets to stop a metahuman that can control gravity, with H.R. and Joe guiding them from S.T.A.R. Labs.

When Grodd and his army of gorillas bring the battle to Earth-1, The Flash and team must find a way to stop them before they destroy Central City. Gypsy returns to join the fight. Meanwhile, Jesse Quick decides she wants to stay with Wally on Earth-1.

Barry and team are surprised when Mon-El and Hank Henshaw arrive on their Earth carrying a comatose Supergirl who was whammied by the Music Meister. Unable to wake her up, they turn to Team Flash to save her. However, the Music Meister surprises The Flash and puts him in a similar coma, one that Team Flash can't cure. Kara and Barry wake up without their powers in an alternate reality where life is like a musical and the only way to escape is by following the script, complete with singing and dancing, to the end.

The crossover starts on Supergirl S02E16 Star-Crossed (I).

Barry and Iris's wedding brings the gang together, but things go awry when villains from Earth-X attack the ceremony. All of the superheroes band together with help from their super friends like Citizen Cold, The Ray, Felicity Smoak, Iris West and Alex Danvers to take on their most formidable villains yet. Earth's mightiest heroes - Green Arrow, Supergirl, The Flash and White Canary - lead their teams into battle to save the world.

The crossover starts on Supergirl S03E08 Crisis on Earth X (I) and Arrow S06E08 Crisis on Earth X (II). It continues on DC's Legends of Tomorrow S03E08 Crisis on Earth X (IV).