When Mr. Mxyzptlk, a magical imp, shows up on Earth and declares his love for Kara, she tries to let him down easy, thinking him harmless. However, he decides it’s a challenge and starts to wreak havoc on National City. Mon-El has seen beings like Mr. Mxyzptlk on Daxam and knows how dangerous they can be. He and Kara argue over how to get rid of Mr. Mxyzptlk, further straining their relationship. Meanwhile, Alex and Maggie celebrate their first Valentine’s Day together.

When Jeremiah Danvers is rescued from Cadmus, Alex and Kara are thrilled to have their father back. The Danvers arrange a family dinner to celebrate but things go awry when a suspicious Mon-El starts to question Jeremiah about his sudden return.

As Alex searches for Jeremiah and Kara investigates a series of alien kidnappings, the sisters realize they must break the rules to foil a new Cadmus plot. In an effort to get Snapper Carr to run a story that would help her stop Cadmus, Kara agrees to set up an interview between Snapper and Supergirl.

A new villain comes to National City, putting Supergirl on high alert. Meanwhile, Winn’s girlfriend, Lyra, gets Winn in trouble with the law. Maggie attempts to help Winn but old loyalties get in the way. The Music Meister attacks Supergirl.

The crossover continues on The Flash S03E17 Duet (II).

Lena’s ex-boyfriend, Jack Spheer, comes to National City to unveil his big breakthrough in nano-technology, which has the potential to eradicate all diseases. Lena asks Kara to attend Jack’s conference with her for support. When Kara sees Snapper in attendance she’s inspired to tap back into her reporter skills and questions Jack on his discovery, which ultimately leads to a bigger investigation that puts everyone in danger.