Returning home from a camping trip, four friends pick up a stranded motorist, who surprises them all with blast of knock-out gas. Coming to, the four now find themselves imprisoned in a decrepit house with six other strangers, and all are outfitted with a device attached at the base of the skull. Unsure what they are doing there and who is behind it all, the group is soon enlightened by video that advises them that there is a way out of the house. But the threshold they must cross is one of excruciating pain...