Synopsis: Still traveling on his own, the Doctor lands in Melcombe Regis in 1348—"the exact time and place where the black death first appeared in England".
He bumps into a crow-like humanoid, who quickly removes his mask to reveal that he is, in fact, fully human. A doctor from the local village, he's trying to treat plague sufferers and the crow's beak is merely a hearty surgical mask. The disease he's encountered, though, is quickly revealed to be not the Black Plague, but some odd malady that turns its victims into green-skinned zombies.
As the two doctors encounter some locals, the explanation they get is that the "zombies" have upset the local faeries. Not quite believing the supernatural explanation, the Doctor asks to be led to the "faerie nest" that's afflicted the "zombies".
The sonic screwdriver analyzes the nest and determines it to be a spaceship of a race called the Zeerover. The local doctor is overwhelmed by one of the "zombies", and yet his flesh isn't green. The Doctor senses something significant about that, and engages in dialogue with the "zombified doctor".
The latter explains that their spaceship is damaged, and that the only way they can survive is to inhabit the bodies of the humans, who act as filters for earth's atmosphere. Over time, though, the "possession" toxifies the humans, turning them green. The solution would normally be for the Zeerover to use humans for only brief periods of time, but because of the Black Plague, no new humans are coming into the area.
Seeing the untenability of this situation, the Doctor instead makes a bargain with the Zeerover. If they'll repair the humans they've damaged, he'll repair their spaceship and they can continue on their journey.
They agree, the Doctor whips up a quick fix, and history is allowed to continue its grim course. The "Green Plague" can now be replaced by the Black Plague.
Notes: This story marks the first time in the publication's history that the magazine editor wrote the comic strip. It's also a rare example of the one-issue story.