Uhura seems to be the only one who can hear a strange sound. When the noise triggers terrifying hallucinations, she enlists an unlikely assistant to help her track down the source.

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

Paul Wesley (Lt. James T. Kirk)

Bruce Horak (Lieutenant Hemmer)
Dan Jeannotte (Lt. Samuel Kirk)
Michael Reventar (Lt. Saul Ramon)
Carol Kane (Commander Pelia)

Alex Kapp (U.S.S. Enterprise Computer)

• Starfleet has built a deuterium refinery at Bannon's nebula.
  
• Pike is temporarily promoted to Fleet Captain, in charge of the U.S.S. Farragut and deuterium refinery.
  
• James Kirk visits his brother Sam on the Enterprise from the Farragut. He has been promoted to first officer of the Farragut, making him the youngest to ever hold the position. The previous holder of the record was their father George Kirk, when he became first officer of the U.S.S. Kelvin. Sam is frustrated by this because he feels it makes him look bad in the eyes of their father.
  
• Sam is named after his father, but goes by his middle name.
  
• James Kirk meets Uhura, Pike, and Spock for the first time in this episode, as well as La'an face-to-face.
  
• Non-corporeal lifeforms are living in the deuterium, and attempting to communicate with Ramon and then Uhura.
  
• Bruce Horak appears as Lieutenant Hemmer in engineering tutorials he had made with Uhura, as well as vision of the aliens.