Five college friend, Marty, Amber, Noah, Eve, and Amber's boyfriend Wesley, trek into the Sangre De Cristo Mountains in New Mexico for a GPS Internet treasure hunt. While digging in a cave they find a stone box which seemingly has nothing to do with the box of CDs that they're supposed to find. They open it and Marty, a student in ancient cultures, realizes the box is Aztec. Inside they find a pot filled with liquid blood. As Amber prepares to take video of it, she spills the blood on the ground. They look up to realize that the sunlight streaming into the cave is disappearing. hey go outside and discover that the sun has disappeared at 11 a.m. in the morning.
The friends listen to the radio in horror as newscasters describe how the sun has disappeared and temperatures are dropping across the world. Marty has been studying the box and explains that they're responsible. The Aztecs performed an ancient ritual to maintain the sun. When the blood was spilled, the sun disappeared. Without the sun, all life on Earth will freeze to death. However, Marty notes that the pictographs reveal only one way to bring the sum back: a human sacrifice. He's found a knife in the box. The other four think Marty is insane and Eve encourages them to go out and find wood for a fire. However, once they're gone, she admits to Marty that she isn't as confident as she made out.
Over the next thirteen hours the global death toll rises while the friends huddle in their cave around a fire. Amber thinks she's the one responsible since she spilled the blood. The night passes but the sun still hasn't appeared. Wesley refuses to participate and the others debate how to pick the sacrificial victim. Amber wonders how they can possibly live with themselves after killing one of their own. Despite that, she agrees to participate and convinces Wesley to join them. Noah grabs a knife and threatens each of them, asking how they can do it. However, he collapses from the cold and they realize there's no more time left.
They prepare four long and one short twig and draw lots, the men first. Each one draws a long twig and finally only Amber and Eve are left. Amber draws and gets the short twig. She tries to delay things, asking for more time to write a final note for her mother. Noah finally hits her over the head with a rock and then holds her down while Marty takes the knife. Wesley refuses to participate or stop them, and Eve looks on in horror as Marty delivers the killing blow.
Later, Marty places the refilled pot in the box and reburies it where they found it. Wesley is sitting over his girlfriend's corpse and blaming himself. As soon as the box is back in place, the sun appears once more. They go outside and turn on the radio. The newscaster says that scientists have determined that a stellar cloud temporarily blocked the sun, and the friends are left to wonder if they did the right thing, or if they even needed to do anything and it was all some cosmic coincidence.