An accident with an experimental quantum probability field causes everyone on the U.S.S. Enterprise to break uncontrollably into song, but the real danger is that the field is expanding and beginning to impact other ships — allies and enemies alike.

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)

Melanie Scrofano (Captain Marie Batel)
Dan Jeannotte (Lt. Samuel Kirk)
Bruce Horak (General Garkog)
Carol Kane (Commander Pelia)

Rong Fu (Lt. Jenna Mitchell)

• A special musical episode, "Subspace Rhapsody" will feature 10 original songs, plus a "Subspace Rhapsody" version of the series’ main title, with music and lyrics by Kay Hanley and Tom Polce.
  
• James Kirk arrives on the U.S.S. Enterprise to shadow Number One for first officer training.
  
• Nurse Chapel is accepted into a fellowship program with Dr. Roger Korby, who was featured in the TOS episode "What Are Little Girls Made Of?", where he was revealed to be Christine's fiancé.
  
• Captain Batel's first name is revealed to be Marie.
  
• Klingon General Garkog is played by Bruce Horak, who played Season 1's chief engineer Hemmer.
  
• La'an reveals to Jim Kirk that he met an alternate time version of him and they had feelings for him.
  
• Kirk reveals that he is in a relationship with a scientist named Carol, who is pregnant. We meet future Carol Marcus and their son David in Star Trek II: The Wrath Of Khan.