CW | 8:00 pm

A crossover among all four of The CW’s superhero shows begins with "Medusa," as a Thanksgiving visit from Kara and Alex’s (Melissa Benoist, Chyler Leigh) mother (guest star Helen Slater) sets off revelations and confessions. Kara teams up with Lena Luthor (Katie McGrath) to stop an alien-killing virus, then joins forces with "The Flash’s" Barry Allen and Cisco (guest stars Grant Gustin, Carlos Valdes) to combat hostile, invading aliens.