Medusa (1)

Eliza comes to town to celebrate Thanksgiving with her daughters. Alex decides it’s time to come out to her mother; Kara is shocked when Eliza suggests Mon-El has feelings for her, and Winn and James 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. Barry Allen and Cisco Ramon seek Kara’s help with an alien invasion on their Earth.
Part 1 - Supergirl - S02E08 "Medusa"
Part 2 - The Flash - S03E08 "Invasion!"
Part 3 - Arrow - S05E08 "Invasion!"
Part 4 - DC's Legends of Tomorrow - S02E07 "Invasion!"