|Supergirl realizes that in order to stop Lex and Leviathan she must work with the one person she never thought she’d trust again – Lena. Nia keeps dreaming about Brainy but struggles with what the dreams are trying to tell her. Meanwhile, Brainy realizes there is only one way to stop Lex.|
- Melissa Benoist as Kara Danvers/Supergirl, J'onn J'onzz (as Kara Danvers/Supergirl) and M'gann M'orzz (as Kara Danvers/Supergirl)
- Chyler Leigh as Dr. Alex Danvers
- Katie McGrath as Lena Luthor
- Jesse Rath as Querl "Brainy" Dox/Brainiac 5
- Nicole Maines as Nia Nal/Dreamer
- Azie Tesfai as Dr. Kelly Olsen
- Andrea Brooks as Eve Teschmacher
- Julie Gonzalo as Andrea Rojas/Acrata
- Staz Nair as William Dey
- David Harewood as J'onn J'onzz/Martian Manhunter and Kara Danvers/Supergirl (voice only)
Special guest starring
- Brenda Strong as Dr. Lillian Luthor
- Sharon Leal as M'gann M'orzz/Miss Martian and Kara Danvers/Supergirl (voice only)
- Lynda Boyd as Eve Teschmacher's mother
- Mitch Pileggi as Rama Khan
What Lex Luthor stole from the Fortress of Solitude were three rings that need Kryptonite to empower them. When Lex visits the true headquarters of Leviathan, which emits specific radiation that kills all but Jarhanpurians, Rama Khan assures him that he, Sela and Tezumak can handle Supergirl. They know that they can track her wherever she goes or uses her powers.
The Superfriends know this, too, and extensively discuss it after they kick in the door to Kara Danvers' apartment thinking that Lena Luthor was a villain. They decide that while Lena makes Supergirl a protective suit, J'onn J'onzz and M'gann M'orzz will impersonate her. The ancient immortals fall for the ruse and attack the Superfriends.
Kelly Olsen cannot find William Dey and alerts Alex Danvers who now sports her new suit and has learned how to work her Martian weapon. With the help of Lena, whom Kara has angrily reprimanded for her past actions, they find him as a prisoner of Eve Teschmacher, who has little problem with killing him to protect her mother. Supergirl calms the situation but Leviathan attacks, using their Kryptonite. Suddenly, the suit arrives and protects the hero. She causes an explosion that destroys the immortals, but they reintegrate.
Querl Dox, on his own, invents an idea to bottle the ancients, to use his people's technology for good. Brainy enters the spacecraft base, to his detriment, and begins a program with their computer to defeat the ancients.
Supergirl learns from Eve that virtual reality will be humanity's mental graveyard if Lex and Leviathan have their way, so she enters the realm to have people end their simulation. Gamemnae grows tired of Supergirl's interference; she sends Andrea Rojas to deal with her, citing that she has always been Andrea's protector. When Andrea hesitates to follow directions, Gamemnae, having revealed her leadership of the covert organization, threatens that she will assure that Bernardo Rojas will not survive. Andrea, as Acrata and carrying a Kryptonite knife, finds Lena protecting the helpless Supergirl. Lena talks Andrea out of her plans and Supergirl talks billions out of using Obsidian North.
The Superfriends begin to lose their battle against the ancient elementals when they suddenly disappear; the dying Brainiac forcibly teleported them and shrunk them into a bottle.
Gamemnae, a goddess of technology, is able to resist the trap, through great difficulty and struggle. Lex sees what is occurring and immediate goes to the ship. He once again laughs at Brainy; Lex calculated for this, too. He takes the bottle and, with the Transmatter portal, goes to see Lillian Luthor.
With Leviathan gone, Nia Nal has time to dream; she sees that Brainy is dying, crying and alone.
After she reunites Eve with her mother, Kara and Lena reconcile and decide that Lex is the last villain they must defeat.
Lex hands Lillian the bottle. She smiles as he tells her to get to work.
- The title is a pun on Mortal Kombat. In addition, this episode is the second Arrowverse episode to reference the video game franchise as an episode title. The first was the episode "".
- Supergirl's rescue of Eve Teschmacher's mother was a reference to the 1978 Superman movie, where Superman convinced Teschmacher to help him in exchange for him saving her mother.
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