2-01: Amok Time
When Spock exhibits irrational behavior from pon farr, The Enterprise heads to Vulcan for help.
2-20: Return to Tomorrow
Several crew members are possessed by advanced non-corporeal beings.
2-23: The Omega Glory
The Exeter is found drifting in space, its crew dead from a mysterious disease, except its missing captain.
2-04: Mirror, Mirror
A transporter accident swaps Kirk, McCoy, Scotty, and Uhura from distinctly opposite parallel universes.