A mysterious sphere threatens the U.S.S. Discovery even as May, in her original form, implements a plan that puts Tilly’s life in danger. Saru and Burnham’s bond grows when Saru is forced to acknowledge a deeply unsettling Kelpien truth. Pike receives new intel on Spock from a loyal friend.

Sonequa Martin-Green (Commander Michael Burnham)
Doug Jones (Commander Saru)
Anthony Rapp (Lt. Cmdr. Paul Stamets)
Mary Wiseman (Ensign Sylvia Tilly)
Anson Mount (Captain Christopher Pike)

Tig Notaro (Cmdr. Jett Reno)
Rebecca Romijn (Number One)
Rachael Ancheril (Commander Nhan)
Bahia Watson (May Ahearn)

Hannah Cheesman (Lt. Cmdr. Airiam)
Emily Coutts (Lt. Keyla Detmer)
Patrick Kwok-Choon (Lt. Gen Rhys)
Oyin Oladejo (Lt. Joann Owosekun)
Ronnie Rowe Jr. (Lt. Ronald Bryce)
Julianne Grossman (Discovery Computer)
Raven Dauda (Dr. Tracy Pollard)
David Benjamin Tomlinson (Lt. Linus)

• Saru prematurely begins to undergo vahar'ai, the death process, signalling he is ready to be culled by the Ba'ul, the predator species on his homeworld. His inflamed threat ganglia are a symptom. He asks Burnham to sever his ganglia to end his suffering. Before they can complete the ritual, Saru's threat ganglia whither and fall off. They discover that Kelpians are culled by the Ba'ul before this happens to keep them afraid as prey.
  
• The sphere is dying. It is over 100,000 years old and wants to transfer the entirety of its data before it dies.
  
• May is a JahSepp, a sentient species native to the mycelial network. Their ecosystem is dying, which they attribute to the Discovery's spore drive.
  
• This is the first appearance of Rebecca Romijn as "Number One," the U.S.S. Enterprise's first officer. She was portrayed by Majel Barrett in the Original Series unaired pilot "The Cage".