Mon-El exchanges blows with Cyborg Superman in a new preview from Monday's jam-packed episode of "Supergirl."

Titled "Medusa," the Thanksgiving-themed episode serves as both the midseason finale and the prelude to The CW's eagerly anticipated crossover, "Heroes v Aliens," which encompasses "The Flash," "Arrow" and "DC's Legends of Tomorrow."

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In the clip, Chris Woods' Mon-El sees who he thinks is J'onn J'onzz at the Alien Bar, only to quickly discover his mistake when it turns out to be Hank Henshaw, the Cyborg Superman. Hey, they're both played by David Harewood, so it's an easy error to make. However, it may work out for the best for Mon-El, as while they're fighting in the alley, deadly gas is being released inside -- presumably CADMUS's alien-killing virus.

A NIGHT OF CONFESSIONS; THE EPIC SUPERHERO CROSSOVER KICKS OFF TONIGHT — Eliza (guest star Helen Slater) comes to town to celebrate Thanksgiving with her daughters. Alex (Chyler Leigh) decides it’s time to come out to her mother; Kara (Melisa Benoist) is shocked when Eliza suggests Mon-El (Chris Wood) has feelings for her, and Winn (Jeremy Jordan) and James (Mehcad Brooks) consider telling Kara the truth about the Guardian. Meanwhile, CADMUS unleashes a virus that instantly kills any alien in the vicinity so Kara enlists an unexpected ally – Lena Luthor (Katie McGrath). The epic superhero crossover kicks off tonight when Barry Allen (guest star Grant Gustin) and Cisco Ramon (guest star Carlos Valdes) seek Kara’s help with an alien invasion on their Earth.

That's only the beginning of what's in store in "Medusa," which also sees Melissa Benoist's Supergirl turn to Lena Luthor (Katie McGrath) for help, and Grant Gustin's Barry Allen and Carlos Valdes' Cisco Ramon travel to Kara's Earth to enlist her aid in the fight against the invading Dominators.

iring Mondays at 8 p.m. ET/PT on The CW, “Supergirl” stars Melissa Benoist, Mehcad Brooks, Chyler Leigh, Jeremy Jordan, David Harewood and Chris Wood.