Four men are trekking across the Antarctic on dog sleds on an expedition at the tail end of World War I. The youngest of the four men, Garner, is a doctor who served in the war and whose wife recently passed away from a flu before he could return home to say goodbye. He is mourning her and came on this expedition for the solitude and silence Antarctica offers. 
As they pass through the treacherous territory, their sled is thrown and the lead dogs begin crying out. They have fallen into a deep crevasse. The captain, Connelly, orders Garner to cut the line and let the lead dog fall all the way into the crevasse in order to save the sleds. Garner is tender-hearted and can't bring himself to do it, so Connelly does it himself, feeling annoyed at Garner's softness. 
After they recover from the immediate threat of the crevasse they begin regrouping. Connelly lets them know that two dogs are now lost- the one that fell into the crevasse as well as another with a broken leg from the accident. One of the men, Faber, is already injured and drifting in and out of a morphine haze. Connelly orders Garner to take care of Faber while he decides what they will leave behind to compensate for the missing dogs. 
That night, the lead dog is still crying from down in the crevasse. The men are surprised it survived the fall. Alone, Garner goes down into the crevasse to help the dog. It is badly injured, and he slits its throat to put it out of its misery. He shines his flashlight around to take in the surroundings and is awestruck. There is a staircase carved into the side of the ice, and he senses creatures in the darkness reaching out to him. He utters the name of his deceased wife and feels called farther into the darkness with the creatures attacking him.
Just then, Connelly looks down from the top of the crevasse and begins pulling Garner back up. He is angry at him for taking such a pointless risk. Garner tries to tell him that there is something down in the crevasse, but he will not listen. He shines his flashlight down there to show him, and Garner and Bishop watch something drag the dead dogs body into the shadows. 
Spooked by the inexplicable force pulling the dead dog, they return to camp where Faber has awoken from a morphine nightmare and is panicking. He knocks over a kerosene lamp and the tent catches on fire. They eventually put the fire out and take care of Faber, and head to the place where a ship will pick them up the next morning, ending their expedition a month early. Faber passes away from his injuries, and they put his body in a boGarner tries to get Bishop to talk about what they witnessed in the crevasse, but Bishop denies seeing anything, seemingly wanting to put the horrific night to rest. 
Garner closes his eyes and imagines walking into the tundra alone and descending the stairs he cannot forget.