Commander Trask is in charge of a mission to a planet intended for colonization. His spaceship goes down and he and three of his men survive: Simmons, Cooper, and Boltz. The remaining two crew die in the crash. Mankind has sent drones to the planet to place survival habitats, the Sundomes, but otherwise the planet is shrouded in continual pouring rain. The men have no choice but to make the eight-hour journey to the nearest Sundome, using the tracking computer. As they start to leave, they notice that the plants are slowly moving, enshrouding the dead crewmen.
As they go through the jungle, Trask insists that they'll make it. They wonder if the rain has ever stopped in a million years. They pass a burst of St. Elmo's fire, and Boltz worries that it may be radioactive. They finally spot a metal structure up ahead and run to it, only to discover that it's their crashed spacecraft. They've somehow gone in a circle.
Boltz grabs the tracking computer and throws it to the ground. Trask orders him to pick it up but before they can retrieve it, a lighting storm strikes the area. The men throw themselves to the ground but as the lightning continues, Cooper panics and tries to run. The lightning is drawn to him and he's struck repeatedly and killed. Boltz insists that he's had enough and wants to go into the ruined spacecraft to seek what shelter he can. Trask warns against it and slaps the man when he refuses to obey.
Boltz reluctantly goes with Simmons and Trask as they try to find the Sundome. The constant rain finally drives him insane. He gives up and lies on his back, letting the constant rain fill his mouth. Boltz goes catatonic and Trask and Simmons are unable to revive him. He's too heavy to carry, and Simmons considers shooting him now and putting him out of his misery. Trask tells the lieutenant that it's Boltz's choice and they continue.
They travel throughout the night, trying to avoid the slowly grasping plants. In the morning, Simmons finally gives up despite Trask's entreaties. The lieutenant drops to his knees and draws his gun, threatening to shoot Trask if he doesn't leave. Trask insists that they're only an hour away but Simmons says that he's made his decision and Trask will have to make his when the time comes.
As Trask treks through the jungle, he finally sees the Sundome up ahead. As he turns to get Simmons, he hears a single gunshot. The commander staggers toward the welcoming structure as plants clutch at him and he slips in the mud and water. Trask refuses to die this close and finally gets to the Sundome. He goes inside and removes his clothing, bathing in the warm sunlight. He then takes a shower with clean water.
After emerging from the shower, Trask prepares to take 24 hours of rest before moving on. He dozes off but wakes up to discover the dome is leaking. Trask looks around and discovers that the plant life caught on his boot has grown throughout the Sundome, splitting it open and letting the incessant rain in. He screams in terror… and wakes up from his nightmare. He throws the few scraps of plant life from his clothing out into the rain and tells his dead comrades that the rain will never get in.