The first in a four part story which concludes Peter Davision's reign as the Doctor.
Landing on Androzani Minor, the Doctor and new assistant Peri explore some nearby cave systems. They are accidentally infected by Spectrox Toxemia which they do not realise will cripple and soon kill them.
Some gun runners ambush the army personel in the caves - they have been supplying arms to the armies enemy, android leader Sharez Jek to which the war still continues. The Doctor and Peri are blamed for the ambush and placed in a cell awaiting execution.
Come morning, the Doctor and Peri are tied up and shot...