An astronaut guarding the last the Earth's forests defies an order to destroy his precious cargo.

In the future, Earth has lost all of its vegetation due to neglect and pollution, with the only remaining specimens carefully tended to in huge geodesic domes attached to several far-roving space freighters. Each ship maintains different ecosystems, with the U.S.S. Valley Forge sustaining six large forests, complete with woodland animals. Freeman Lowell, the most dedicated member of the project, has been on the Valley Forge for eight years and fervently believes that Earth will be repopulated with trees, flowers, vegetables and fruits, despite the derision of his fellow astronauts, John Wolf, Marty Barker and Andy Keenan. The other men, younger than Lowell, refuse to eat the produce he grows, instead preferring synthetic foods manufactured by the kitchen units. Maintenance aboard the vast ship is carried out by three small drones, each equipped with short legs, a mobile arm that can use a variety of tools and a camera through which the crew can see what the drone sees. Lowell frequently bickers with the other men, especially Barker and Keenan, who assert that if anyone on Earth really cared, the reforestation program would have been taken more seriously years earlier. Despite their antagonism, the men often relieve their boredom by playing poker, at which Lowell usually wins. One day, Lowell's worst fears are realized when the group receives orders that all of the domes are to be exploded and the ships returned to commercial service, with their crews to report back to Earth. Although the other astronauts are thrilled by the prospect of returning home, Lowell glumly goes to his gardens to save a few flowers. Wolf reminds Lowell that there is no more poverty or unemployment on Earth, as well as very little disease, but Lowell retorts that there is also no more beauty, imagination or frontiers to conquer, and that no one cares any longer. Wolf, Barker and Keenan begin placing the nuclear bombs into the domes and preparing to jettison them, while Lowell becomes more agitated. His anger increases as he watches the domes explode, until finally, when Wolf enters the dome in which Lowell is working, Lowell blocks his path, insisting that he cannot destroy the irreplaceable forest. In the ensuing struggle, Lowell's leg is badly injured but he succeeds in strangling Wolf. Limping to the control room, Lowell then jettisons and explodes the dome in which Barker and Keenan are working, thereby killing them also. When the lead ship, the Berkshire , inquires about the Valley Forge , Lowell responds that the other men have been killed in an accident and the ship has been damaged by an explosion, which he simulates by having the drones throw cargo pods out into space. Lowell then sets a course for the outer rings of Saturn and tells Anderson, the squad commander, that he cannot repair the damage nor change course. Anderson tells Lowell that he will probably die in the turbulence as the ship passes through the rings but promises to send a search party. Satisfied that he has eluded the others while saving one dome, Lowell then reprograms the three drones so that they can operate on his leg. With the drones' help, Lowell survives the surgery and falls asleep. He is awakened by a fierce shaking as the ship passes through Saturn's rings and, desperate, watches the monitors showing the progress of the three drones, who are on the outside of the ship. He orders them inside and watches with great dismay as the third drone does not make it in time and is ripped away by the storm. The other two reach safety, however, and soon the intact ship floats in the quiet of the other side of Saturn. Lowell is thrilled by the success of his plan but troubled by Wolf's body, which he orders the drones to bury. Although he regrets having resorted to violence, Lowell asserts that he had no other choice. Lowell then reprograms the drones again so that they will answer directly to him at all times. He names the smaller, gray one Dewey and the larger, orange one Huey, while noting that the third one, who would have been Louie, is no longer with them. As their new programming enables them to work closely with Lowell in caring for the forest, the drones also become less robotic in their behavior, following Lowell like pets and responding to him with squeaks and flashing lights. Despite their company, Lowell begins to feel lonely and soon is racing around the ship in a motorized cart, wandering aimlessly and sadly remembering his companions. Determined to ease his pain, Lowell programs Huey and Dewey to play poker and although the drones cheat by showing each other their cards, Lowell laughs with delight when Huey wins. Lowell's malaise continues, however, until one day, Huey and Dewey silently stare at him while he prepares a meal of synthetic food. Feeling their reproach, Lowell admits that he has not been eating real food and jogs to the forest, which he has been neglecting. To his astonishment, the plants are turning brown and dying, and none of his research reveals an answer to the problem. As he continues to investigate, he orders Huey, who is in the forest examining specimens, to remain there while he joins him. Huey is waddling down the corridor, however, when Lowell races through on a cart and smashes into him. Although he tries to repair the damage, Lowell cannot fix Huey, who remains dented, limping and unable to use his arm fully. Grief-stricken, Lowell apologizes to Huey and Dewey and attempts to comfort them. Later, Lowell is awakened by a radio call from the Berkshire , which has successfully maneuvered around Saturn and found the Valley Forge . Lowell is horrified that he has been discovered, especially when Anderson states that he can leave the forest to die, as they are too far from the sun for it to survive, and that they will be picking him up in six hours. Finally realizing that lack of sunlight is causing the forest to wither, Lowell quickly erects a series of bright, artificial lights. With the lights in place, Lowell addresses "the boys," telling Dewey that he has all the knowledge he needs to maintain the forest forever. After tearfully bidding Dewey farewell, Lowell takes Huey, who is too injured to help Dewey, back to the main ship. Lowell then jettisons the dome, as he knows that it will never be looked for if the others believe that the Valley Forge is gone. Setting up a series of nuclear bombs, Lowell pets Huey one last time, then blows the ship up. Later, far away in outer space, the forest dome flourishes with renewed vigor and Dewey waters a young tree with Lowell's favorite watering can.