Duncan McKenzie is a marine biologist working on a small, sparsely populated island in the Scottish Outer Hebrides. The island has an indigenous population of only six people except for a team of archeology students who are opening up what seems to be an early Neolithic burial mound. Two lighthouse keepers, Dick (a young trainee), and Tom (an old hand with many years of experience), are disturbed in their work by the appearance of a dazed female student, badly traumatized and bruised. The student tells of the slaughter of the rest of her party by something that they have released from the burial mound. Soon everyone the three men know is either missing or dead and there are things moving in the fog. Large, hulking, unholy things with a taste for human flesh. Then the fun starts in earnest.