When the planet is threatened by the invasion of a hostile alien species Barry rallies his friends for a superhero team up, including Team Arrow, the Legends, and Supergirl. But as things go from bad to worse, the team is fractured by revelations about Barry’s past actions.

U.S. air dates are in MM.DD.YY form. U.K. air dates are in DD.MM.YY form.

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IT'S HEROES VS. ALIENS IN THIS EPIC CROSSOVER EVENT

When aliens called the Dominators attack Central City, Barry (Grant Gustin) speeds over to Star City to ask the Green Arrow (guest star Stephen Amell) 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 (Carlos Valdes) create a breach to bring back a secret weapon - Supergirl (guest star Melissa Benoist).

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 (Keiynan Lonsdale) wants to join the battle against the aliens but Iris (Candice Patton) tells him he's not ready.

Dermott Downs directed the episode with story by Greg Berlanti & Andrew Kreisberg and teleplay by Aaron Helbing & Todd Helbing (#308).