In the shuttlecraft Copernicus, Spock, Uhura, and Sulu are en-route to
Starbase 25 to deliver a stasis box -- a rare artifact of the Slaver culture.
The now-extinct Slavers used these objects to carry weapons, valuables, and other scientific
instruments and data. The boxes can detect each other and evidence shows
that another device is located nearby. The shuttle lands on an ice planet where hostile Kzinti
lie in ambush. Written by Larry Niven, and based on his short story "The Soft Weapon",
it crosses Trek into his 'KnownSpace'.
This is the only episode of "Star Trek" that Captain Kirk does not appear.