The U.S.S. Enterprise encounters a contagion that ravages the ship. One by one, the entire crew is incapacitated except for Number One, Una Chin-Riley, who must now confront a secret she’s been hiding as she races to find a cure.

Anson Mount (Captain Christopher Pike)
Ethan Peck (Lieutenant Spock)
Jess Bush (Nurse Christine Chapel)
Christina Chong (Lt. La'an Noonien-Singh)
Celia Rose Gooding (Cadet Nyota Uhura)
Babs Olusanmokun (Dr. Joseph M'Benga)
Melissa Navia (Lt. Erica Ortegas)
Bruce Horak (Lieutenant Hemmer)
Rebecca Romijn (Lt. Commander Una Chin-Riley)

Sage Arrindell (Rukiya M'Benga)
Andre Colquhoun (Probasco)
Daniel Gravelle (Ensign Lance)
Alex Kapp (U.S.S. Enterprise Computer)
André Dae Kim (Chief Kyle)
Curtis Legault (Carter)

• The U.S.S. Enterprise investigates an abandoned Illyrian colony on Hetemit IX. Illyrians are a humanoid species known for modifying their genes to enhance their capabilities.
  
• (Enterprise NX-01 encounted an Illyrian ship in the Delphic Expanse in the episode "Damage".)
  
• Number One reveals that she is an Illyrian and has genetic augmentations. She tries to resign, but Pike rejects it.
  
• It's revealed that La'an is descended of Khan, but she is not an augment.
  
• The virus got past the transporter's biofilters because Dr. M'Benga didn't allow the emergency medical transporter to be upgraded during the last refit. He is secretly holding his terminally ill daughter Rukiya's pattern in the buffer. Number One in turn does not accept his resignation.
  
• Babs Olusanmokun (Dr. M'Benga) and Melissa Navia (Lt. Erica Ortegas) have swapped positions in the opening credits, with Babs Olusanmokun now appearing before Melissa Navia.