Eliza throws Alex and Maggie a wedding shower, which causes Maggie to reach out to her estranged father. Supergirl joins J’onn on a personal mission.

Kara investigates a secretive new group whose leader, Thomas Coville, has a mysterious connection to Supergirl. Meanwhile, Samantha feels like she's letting Ruby down, and J'onn confesses an old secret.

When Winn and the team discover an alien ship has crash landed deep underwater beneath National City, Kara/Supergirl is called in to investigate. Meanwhile, Hank attempts to bond with his father, Myr'nn. Samantha, eager for some understanding of the changes she has been experiencing, looks to her estranged mother, Patricia Arias, for answers.

Supergirl investigates a mysterious symbol popping up all over National City, tracing its origins back to an ancient prophecy and the mark of the World Killer, Reign. Meanwhile, the tension between Lena and Morgan Edge continues to build, causing James to step in and offer up some protection, and possibly igniting a spark in the process.

When Winn’s father dies, his estranged mother, Mary, reappears trying to reconnect and to explain why she abandoned him all those years ago. Winn is too embittered from his rough childhood to forgive her but when a copycat Toyman attacks, the two must work together to stop him. Mon-El tells Kara something important about the Worldkillers, and Alex becomes suspicious of Myr'nn’s recent behavior.

Myr'nn inadvertently causes psychic disturbances at the DEO; Supergirl must work with J'onn to contain the chaos; Mon-El begins training Kara in fighting techniques he's learned in the future.

When Reign starts hunting Ruby, Supergirl and Alex work together to keep her safe. Supergirl and J'onn look to Sam’s mother for advice on how to stop Reign and come away with some shocking news.

When Tanya, a disciple of Coville's, escapes from what's left of his cult, she gives Kara and James a journal that could hold the key to saving Sam. Someone learns Guardian's identity and threatens to expose James if he doesn't do what they ask. James contemplates what it means to be a black vigilante without a mask in America.

Kara makes a major life decision. Meanwhile, J'onn finds out that special DEO-caliber guns have hit the streets of National City.

Supergirl learns the true depth of Serena's nefarious plans for Earth. Supergirl, Mon-El and Alura must devise a plan to stop her before Serena gets to Earth. J'onn prepares to say goodbye to his father.

Supergirl and team take on Selena in an epic battle for Earth.

Supergirl must deal with the destructive aftermath of Lex's nefarious plans. Shocked at what Lex has created, Supergirl faces her biggest challenge yet.

As Kara doggedly works as a reporter to clear Supergirl's name, Dreamer picks up the slack as National City's protector, which leads to a showdown with Ben Lockwood. James takes drastic measures to relieve his PTSD.

Kara attempts to mend her relationship with Lena. J’onn J’onzz takes a deep dive into his memories while Kelly tries to help an old friend.

Supergirl finally learns the truth about Lena while tangling with Leviathan. J'onn J'onn makes a discovery about his brother's whereabouts.