The U.S.S. Discovery crew journeys to a mysterious planet in hopes of finding a cure for Georgiou's deteriorating condition. Stamets and Adira make a stunning breakthrough with the newly acquired Burn data.

Sonequa Martin-Green (Commander Michael Burnham)
Doug Jones (Captain Saru)
Anthony Rapp (Lt. Cmdr. Paul Stamets)
Mary Wiseman (Ensign Sylvia Tilly)
Wilson Cruz (Dr. Hugh Culber)
David Ajala (Cleveland Booker)

Michelle Yeoh (Philippa Georgiou)

Oded Fehr (Admiral Charles Vance)
Blu del Barrio (Adira Tal)
Hannah Cheesman (Lt. Cmdr. Airiam)
David Cronenberg (Kovich)
Paul Guilfoyle (Carl)
Rekha Sharma (Cmdr. Ellen Landry)

Emily Coutts (Lt. Keyla Detmer)
Patrick Kwok-Choon (Lt. Gen Rhys)
Oyin Oladejo (Lt. Joann Owosekun)
Ronnie Rowe Jr. (Lt. Ronald Bryce)
Sara Mitich (Lt. Nilsson)
Julianne Grossman (Discovery Computer)
Hannah Spear (Dr. Issa)
David Benjamin Tomlinson (Kelpien Servant)

• On Dannus V, Burnham and Georgiou find a mysterious door and a man named Carl, who tells them Georgiou's answers are on the other side. When Georgiou goes through the door, she finds herself back in the Terran mirror universe on the day the I.S.S. Charon was christened — which is also the day she discovered Lorca and Burnham were plotting against her.
  
• Rekha Sharma (Cmdr. Ellen Landry) makes a return as her mirror counterpart on the I.S.S. Discovery.
  
• Hannah Cheesman (Lt. Cmdr. Airiam) also appears as her mirror human self, not augmented.
  
• Both Michael Burhman and Dr. Hugh Culber's mirror counterparts make an appearance in this episode. (They did not actually appear in Season 1's mirror episodes.)
  
• Hannah Spear appears in the distress signal as the Kelpian Dr. Issa on the K.S.F. Khi'tei. She previously played Saru's sister, Siranna, in the second season.