Cyborg Superman unleashes a virus in a bar, killing all aliens there except Mon-El, who is quarantined by DEO. Eliza, visiting for Thanksgiving, determines that the virus is Kryptonian. At the Fortress of Solitude, Kara learns that Henshaw was after "Medusa", a virus created by her father, Zor-El, to kill non-Kryptonian in case of alien invasion. At L-Corp, she stops Henshaw from getting the isotope required to disperse Medusa. Lena refuses to help when Kara reveals that Lillian is Cadmus' leader. Later, Lena willingly gives Lillian the isotope. Kara and J'onn confront the Luthors and Henshaw before Medusa is launched via rocket. J'onn fights Henshaw, becoming a White Martian, while Kara chases the rocket, which explodes and disperses Medusa. However, Lena has double-crossed Lillian and rendered the virus inert. Lillian is arrested but Henshaw escapes. Eliza cures Mon-El and uses Medusa to restore J'onn's Green Martian form. Alex comes out to Eliza, who supports her. Later, Maggie and Alex kiss after Maggie admits she does have feelings for Alex. An unknown alien race is searching for Mon-El. Barry Allen and Cisco Ramon arrive from another Earth to enlist Kara's help.
Cyborg Superman unleashes a virus in a bar, killing all aliens there except Mon-El, who is quarantined by DEO. Eliza, visiting for Thanksgiving, determines that the virus is Kryptonian. At the Fortress of Solitude, Kara learns that Henshaw was after "Medusa", a virus created by her father, Zor-El, to kill non-Kryptonian in case of alien invasion. At L-Corp, she stops Henshaw from getting the isotope required to disperse Medusa. Lena refuses to help when Kara reveals that Lillian is Cadmus' leader. Later, Lena willingly gives Lillian the isotope. Kara and J'onn confront the Luthors and Henshaw before Medusa is launched via rocket. J'onn fights Henshaw, becoming a White Martian, while Kara chases the rocket, which explodes and disperses Medusa. However, Lena has double-crossed Lillian and rendered the virus inert. Lillian is arrested but Henshaw escapes. Eliza cures Mon-El and uses Medusa to restore J'onn's Green Martian form. Alex comes out to Eliza, who supports her. Later, Maggie and Alex kiss after Maggie admits she does have feelings for Alex. An unknown alien race is searching for Mon-El. Barry Allen and Cisco Ramon arrive from another Earth to enlist Kara's help.