Dodo dooms mankind with a sneeze; cyclopses lock humans in a security kitchen; and incorporeal creatures sit down in chairs.
The Doctor, Steven and Dodo visit an Ark in space (not that one), where most of mankind has been miniaturised and put on shelves in cold storage. The few regular-sized humans around are aided by a race of cyclopses, called the Monoids, who may or may not be their slaves, and whose origins and presence on the ship are never divulged.
All seems fine until Dodo develops a 5-minute cold and sneezes, and subsequently infects every biped onboard with a deadly plague. It’s now up to The Doctor to cure the common cold and save the day.
But that’s not all – and let’s keep this as spoiler-free as possible – hm, were the troupe to leave and return, say, some centuries later, then it would potentially be up to them to rescue said humans from, say, a security kitchen.