Tonight, The CW's four-night crossover began on "Supergirl" when Barry and Cisco showed up to ask Kara for help. The long-awaited crossover episode will continue tomorrow on "The Flash" in "Invasion: Part 2."

This new promo showcases Barry's challenge to Supergirl to see who is faster, as well as the team coming together to fight the alien race, The Dominators. The promo also sets up The Dominators powers of mind control, which may put some of the team against one another, including Thea, who humorously thinks she can shoot The Flash with her bow and arrows.

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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). Original airdate 11/29/2016.

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Starring Grant Gustin in the title role, “The Flash” airs Tuesdays at 8 pm ET/PT on The CW. The series also stars Jesse L. Martin, Tom Cavanagh, Carlos Valdes, Candice Patton, Danielle Panabaker, Keiynan Lonsdale and more.