This Western is set in the Mexican valley of Sonora not that far south of the state of Arizona. It's just after the American Civil War, and a former Confederate officer, Mike Summers has taken refuge in a small town in the valley. He has married and is hoping to live in peace the rest of his life. Instead, he and his wife and the rest of the town are suffering the depredations of a brutal gunman, Danny Pose, and his gang of outlaws. Summers holds off picking up a gun because of his personal vow of non-violence. But the situation deteriorates and a new ally comes into the picture, Steve Fallon, a wandering gunslinger who may not be able to handle the bad guys alone.

The Doctor, Ian, Barbara and Susan arrive in the TARDIS on board a spaceship. Their initial concern is for the ship's human crew, who are suffering from telepathic interference from the Sensorites, but Susan communicates with the Sensorites and finds the aliens fear an attack by the humans and are just defending themselves. Travelling to the Sense Sphere (the Sensorites' planet) the Doctor seeks to cure an illness to which the Sensorites and Ian have succumbed, but finds it has been caused by deliberate poisoning. The political manoeuvring of the Sensorite City Administrator poses another threat to the TARDIS crew as he seeks to discredit and implicate them.

Set during the Burma Campaign of World War 2, this is the story of courage and endurance of the soldiers struggling at close quarters against the enemy. The film examines the moral dilemmas ordinary men face during war, when the definitions of acceptable military action and insupportable brutality become blurred and distorted.