A freezing night, an artillery barrage that's coming too close, a patient going downhill, and Frank's searching Hot Lips' tent for his letters.

The edge of the known universe, in the year 37,166. The TARDIS lands on the planet Zeta Minor, where a Morestran survey team has suffered a series of horrific killings. What dark secret does the team leader keep - and will the planet permit anyone to leave?