|Season 3, Episode 17|
|The Trial of Kal-El||The Swarm|
A PLANETARY CELEBRATION- With the occurance of Nova Day, an annual religious holiday where all crime can be absolved by the Mediator Guild, the trial ceases. During the festivities, Zod's assassin strikes. Karen informs Kara that Lena stowed away on Astra's ship. Kal-El shows compassion for an enemy. Lesla-Lar returns to New Krypton with a deadly agenda, while Alura finally falls out with her family.
- Melissa Benoist as Kara Danvers/Supergirl
- Chyler Leigh as Alex Danvers (credit only)
- Jeremy Jordan as Winslow Schott (credit only)
- Katie McGrath as Lena Luthor
- Laura Benanti as Alura Zor-El/Astra In-Ze
- Tyler Hoechlin as Kal-El
- Floriana Lima as Maggie Danvers (credit only)
- Zachary Quinto as General Dru-Zod
- Odette Annable as Reign
- Jenna Dewan as Lucy Lane (credit only)
- with Claire Holt as Karen Starr
- and David Harewood as J'onn J'onzz (credit only)
- Gregg Henry as Ral-Dar
- Georgina Campbell as Lieutenant Asha Del-Nar
- Harry Lennix as Commander Fer-Gor
- Tracy Spiridakos as Lesla-Lar
- Michael Rosenbaum as LX-13
- Dianna Agron as Raya Zod-El
- Paige Turco as Lara Lor-Van
- Casper Crump as Vandal Savage
- Madison Riley as Lieutenant Tara Ky-Gor
- Alessandra Torresani as Young Jindah Kol Rozz (flashbacks)
- Genevieve Buechner as Young Astra In-Ze (flashbacks)
- Robert Gant as Zor-El (hologram)
- Wallis Day as Nyssa-Vex
- and Emmy Rossum as Zaora Dru-Zod
- Antagonists: Ral-Dar, Lesla-Lar
- Alex, Maggie, Sam, Lucy, and J'onn do not appear in this episode.
- 3rd episode this season in which Earth does not appear.
- Lena and Kara interact for the first time since Brave Enough.