La'an travels back in time to twenty-first-century Earth to prevent an attack which will alter humanity's future history — and bring her face to face with her own contentious legacy.

Anson Mount (Captain Christopher Pike)
Ethan Peck (Lieutenant Spock)
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)

Adelaide Kane (Sera)
Carol Kane (Commander Pelia)

Dennis Barham (Chess Player #1)
Noah Lamanna (Chief Jay)
Caeden Lawrence (Security Ensign)
David Leyshon (Chess Player #3)
Luke Marty (Police Officer)
Sean Meldrum (Denobulan Cadet)
Brian Quintero (Store Security Guard)
Robin Schisler (Woman in Store)
Randy Singh (Institute Guard #1)
Desmond Sivan (Young Khan)
Mish Tam (Chess Player #2)
Allison Wilson-Forbes (Agent Ymalay)
Christopher Wyllie (Grey-Suited Man)

• The "Grey-Suited Man" is a Department of Temporal Investigations agent. When he is shot in the past, he travels to the U.S.S. Enterprise and gives La'an his time travel device. When he dies, the timeline changes. Kirk is now captain of the "United Earth Fleet" ship U.S.S. Enterprise and Earth is at war with Romulus.
  
• The Vulcans are also at war with Romulus and losing. Captain Spock of the starship Sh'Rel requests Kirk's help to save the last of the Vulcan fleet, which he denies.
  
• In this timeline, Kirk was born on the starship U.S.S. Iowa, not on Earth, which is a wasteland. Humans live in underground cities on the moon. He also does not know who Khan is, which catches La'an off guard.
  
• The device takes La'an and Kirk back to 21st century Toronto, Canada to fix the timeline. Shortly after arriving, a photonic bomb destroys the Lake Ontario Bridge.
  
• La'an seeks out Pelia, who is living on Earth in Vermont at this time as a shady antiques dealer. She gives them a watch that can detect cold fusion byproducts.
  
• La'an and Kirk initially think they are there to stop the destruction of the cold fusion reactor by the Romulans, but they are actually there to stop the killing of young Khan Noonien-Singh by a Romulan operative.
  
• Kirk ends up sacrificing himself in the past to protect La'an. When La'an returns to the future, she contacts the U.S.S. Farragut and speaks to Lieutenant Kirk.
  
• The Department of Temporal Investigations of was initially named in the Deep Space Nine episode "Trials and Tribble-ations".
  
• Jess Bush (Christine Chapel) does not appear in this episode and her name does not appear in the opening titles.