When the TARDIS is buffeted by "time slippage", the Doctor experiences a terrible vision of the end of everything. Tracking the source of the disruption, he takes Rory and Amy to what appears to be an English public school in the 1950s. But as the friends are about to discover, there are some very unusual things about Darkstar Academy. For a start the prefects carry guns, and then there is the strange forcefield that surrounds the perimeter. Not to mention the foot-long, crab-like creatures with spiny, armoured bodies... When the Doctor learns the truth about the Academy, he also discovers that the whole place is in terrible danger. But with a swarm of carnivorous creatures on the loose, what can he, Amy and Rory do to help prevent a terrible disaster?