"Kara (Melissa Benoist) and Lena (Katie McGrath) head to Stryker’s Island to search for clues on how to defeat Lex. At the prison, Kara tries to befriend a nosy prisoner, Steve (guest star Willie Garson), who may have insight into what Lex was doing, while Lena studies an array of puzzles Lex left behind in the hopes of discovering his whereabouts.

While there, a prison riot breaks out and Kara must find a way to stop the prisoners without revealing her secret identity as Supergirl. Meanwhile, Kelly (guest star Azie Tesfai) encourages James (Mehcad Brooks) to talk to someone for his PTSD, and Alex (Chyler Leigh) faces off against Ben Lockwood (Sam Witwer) after he storms the DEO and acquires some powerful weapons to hunt Supergirl."

Supergirl Season 4 airs on April 21. The new season is in full swing, with plenty of new supporting characters and villains to challenge Kara and friends. We've got everything you need to know about Supergirl season 4, as well as a complete episode guide, all in one place.