The TARDIS lands near an isolated rocky island lighthouse just after a comet strikes the nearby sea and a sudden chilling fog rolls in. Soon after, electrical problems take hold of the generator and one of the keepers dies mysteriously. Rueben, the eldest keeper, thinks it's the return of the legendary Beast of Fang Rock, but the Doctor suspects it's something worse that a legend.

A death scream is heard, yet all are accounted for. Eventually the Doctor finds a body stashed away but hours old, with rigor mortis already settling in, which means there's a chameleon in their midst. Completely cut off from the rest of the world and with an escalating body count, the Doctor wonders just what, exactly, he's locked inside the lighthouse with them all.