Captain Burnham and her crew must go where few have gone before: beyond the Galactic Barrier. Meanwhile, Book learns the truth of what drives Ruon Tarka.

Sonequa Martin-Green (Captain Michael Burnham)
Doug Jones (Captain Saru)
Anthony Rapp (Lt. Cmdr. Paul Stamets)
Wilson Cruz (Dr. Hugh Culber)
Blu del Barrio (Ensign Adira Tal)
David Ajala (Cleveland Booker)

Oded Fehr (Admiral Charles Vance)
David Cronenberg (Dr. Kovich)
Shawn Doyle (Dr. Ruon Tarka)
Chelah Horsdal (President Laira Rillak)
Tara Rosling (President T'Rina)
Annabelle Wallis (Zora)
Hiro Kanagawa (Dr. Hirai)
Phumzile Sitole (General Ndoye)
Osric Chau (Oros)

Emily Coutts (Lt. Cmdr. Keyla Detmer)
Patrick Kwok-Choon (Lt. Cmdr. Gen Rhys)
Oyin Oladejo (Lt. Cmdr. Joann Owosekun)
Ronnie Rowe Jr. (Lt. Cmdr. Ronald Bryce)
Sara Mitich (Lt. Cmdr. Nilsson)
Orville Cummings (Lieutenant Christopher)
David Benjamin Tomlinson (Lieutenant Linus)
Christopher Allen (Orion Guard)

• Adira has returned to Discovery from Trill.
  
• The new DMA is operating at a faster rate than previous.
  
• Comm officer Bryce leaves Discovery to continue working with Kovich.
  
• Tarka reveals he was imprisoned in an Emerald Chain camp ten years ago. He met another scientist named Oros who was secretly designing an interdimensional transporter. Tarka befriended Oros, then betrayed him by telling Osyraa about the device. Tarka escaped from the camp, but had to leave Oros behind. He says he regrets betraying him.
  
• Mary Wiseman (Sylvia Tilly) does not appear in this episode and her name does not appear in the opening credits.