Eleven years ago, the U.S.S. Farragut encountered a deadly cloud creature with vampire-like tendencies. James T. Kirk was on board the Farragut at that time as they entered the region of Tycho IV. The creature killed the ship's commander, Captain Garrovick, and half the crew by draining their red blood cells. On his way to deliver necessary medical supplies, Kirk encounters what he believes is the same blood-sucking entity. He pursues it, against orders, determined to destroy it before it kills again.

James Doohan (Lt. Cmdr. Montgomery Scott)
Stephen Brooks (Ensign Garrovick)
Nichelle Nichols (Lt. Uhura)
Jerry Ayres (Ensign Rizzo)
Majel Barrett (Nurse Christine Chapel)
Walter Koenig (Ensign Pavel Chekov)

• Kirk previously served as a lieutenant on the starship U.S.S. Farragut NCC-1647.
  
• Vulcan blood is copper based, not iron as with humans. This gives it a green color.