Two military agents are taking readings on Mount Lincoln near the town of Ascension. There is a quake and the woman dies with marks on her face. The man also dies though we don't see it. Elsewhere, a man is injured in a work crew and dies for no apparent reason. It later turns out the quakes on the mountain are producing a layer of carbon dioxide gas so in laying him down, he suffocated while they did not notice it while standing.
Emily who is studying Mt Lincoln (known for its many quakes) from nearby has to visit the mountain as two "boxes" are malfunctioning. In town we see Thomas's father William who is considered a little dotty as he has some strange theories about the mountain and a Mayan monster which will redress the balance if humanity goes too far. Emily bumps into Thomas in town and they are old friends. Satellite images show movement under the mountain and they believe there are some kind of major quakes and maybe volcanic action going on there.
While everyone else is keeping away from the mountain, Thomas's sister Grace and her boyfriend decide to go camping there, where he plans to propose to her. Thomas is persuaded to go up the mountain as guide for government man, Jack Murray because of something very important there. They meet Emily who is running because part of a mountain suddenly blew out near her, close by. Meanwhile, the proposal was rather ruined by the sight of a monstrous eye appearing out of the side of the mountain and a massive tentacle trying to kill them.
Emily has persuaded the Sheriff to evacuate the whole town but Thomas and Emily decide to go back onto the mountain to look for the missing couple. They find Jack injured on the mountain then have their own close encounter with the monster, who kills Jack but not before he has told them that the troubles at Mt Lincoln were felt all over the world six months ago and a missing box may be their only chance against the monster (lost when the agents were killed), so they go looking for it while looking for the couple.
An earthquake seriously damages the town, and the diner William is in, trying to get owner-waitress Zoe to leave, sinks into the ground. The box is recovered but the monster has found the couple first. The boyfriend ends up in the monster's maw but Thomas turns up in time and pulls Grace to safety. 73 minutes into the film, the monster decides it is time to reveal itself, ripping off the top of a mountain to do so. Thomas opens the box and fires a shoulder-launched multi-warhead missile at the monster, with the one which goes down its throat killing it. William and Zoe escape from the diner in a sinkhole and all are reunited.
The location used for the town and surrounding countryside was
Stawamus Chief(Squamish),
British Columbia,
Canada.
