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The four-part superhero crossover event begins tonight on The CW — but the story doesn't really get going until Tuesday.

Tonight's episode of "Supergirl" (7 p.m., CW/Ch. 30) is, for the most part, just your basic episode. At Thanksgiving, Alex decides to come out to her mother, and CADMUS unleashes a virus that instantly kills any alien.

But toward the end of the episode, Barry Allen/The Flash and Cisco Ramon show up and ask Supergirl/Kara Danvers to come back to their Earth (which is in a different dimension) to help them battle an alien invasion.

That story continues Tuesday on "The Flash" and Wednesday on "Arrow" before conclusing Thursday on "DC's Legends of Tomorrow."

Elsewhere on TV …

• NBA (6 p.m., ROOT): Utah Jazz at Minnesota Timberwolves

• NFL (6:15 p.m., ESPN): Green Bay Packers at Philadelphia Eagles

• "CMA Country Christmas" (7 p.m., ABC/Ch. 4): Music stars perform holiday songs; Jennifer Nettles hosts.

• "The Voice" (7 p.m., NBC/Ch. 5): The 10 remaining contestants — including Utahn Belle Jewel — perform.

• "Gotham" (7 p.m., Fox.Ch. 13): The lab working on a cure for the virus is compromised.

• "Mars" (7 p.m., National Geographic Channel): In 2033, the Daedalus crew struggles to find shelter that can provide protection from radiation

• Men's college basketball (7 p.m., P12N): Butler at Utah

• "People of Earth" (7 p.m., TBS): Gina holds a coming-out event for the members of StarCrossed.

• "Lucifer" (8 p.m., Fox/Ch. 13): Charlotte tries to turn Chloe against Lucifer in order to get him to leave Earth.

• "Studio C" (8 p.m., BYUtv): The Charleston Brothers get help from the YouTube channel ``What's Inside.''

• "Jane the Virgin" (8 p.m., CW/Ch. 30): Against Rafael's wishes, Jane takes Mateo to church.

• "Conviction" (9 p.m., ABC/Ch. 4): The team revisits a kidnapping case that Sam prosecutedt.

• "Timeless" (9 p.m., NBC/Ch. 5): As the nation waits news of the first moon landing, Flynn arrives in 1969, intending to rewrite the day's events.