Tunnel in the Sky is a novel by science fiction writer Robert A. Heinlein. In this novel, Rod Walker is taking a survival course required for his ambition to be an Outlands professional. The final exam has arrived, requiring Rod to be placed on an unknown planet and forced to survive either alone or with one team member for four to ten days. Rod is apprehensive, especially in light of some turmoil at home, but is eager to prove he can survive. However, four to ten days quickly turns into weeks, then months, then years. Tunnel in the Sky is an exciting look at what happens to young people when all modern convenience disappears overnight, making man the greatest danger, as well as the greatest asset, a young man can face.

Rod Walker arrives at his survival class to discover a bulletin announcing the rules for the final exam. Rod is shocked by the lack of guidelines in the rules and suddenly frightened of a prospect that has seems miles away until this moment. Rod decides to team up with someone, thinking two heads are better than one, but his one friend in the course announces he is considering dropping the course rather than face the final exam. Rod goes home that night, weary of the battle ahead, to find his sister home from the Amazon army for a rare visit. That night, however, Rod discovers the reason for his sister's visit. Rod's father is dying and has elected to have himself and Rod's mother frozen until a cure can be found.

Rod elects to continue with the survival course's final exam, taking his sister's advice by carrying with him two knives, but no guns. Jimmy, Rod's friend, surprises him by arriving moments before the test is to begin. Again they decide to team up, but when Rod arrives through the gate, Jimmy is nowhere to be found. Rod spends the first night on the unknown planet frightened and very aware of the dangers the planet poses to his life. The following day, Rod begins to explore despite a sleepless night and his hit over the head, waking to find that all his supplies, minus one knife hidden under a bandage, have been stolen, including the clothing he had been wearing.

Rod manages to survive for several weeks on his own, slowly coming to realize that the rendezvous that was supposed to take place in four to ten days was long overdue. Rod wonders if perhaps something catastrophic happened on Earth, leaving him and his surviving classmates forever abandoned on this strange planet. While hunting, Rod runs into a young student named Jack. Rod and Jack decide to team up and Rod moves into Jack's cave. The following day, Rod and Jack find Jimmy, half dead, lying in a tree. Rod and Jack take Jimmy back to the cave and nurse him back to life. It is at this point Rod learns Jack is a girl.

Rod, Jack, and Jimmy decide to build a signal fire and lure more survivors out of the woods. The next day, Bob and Carmen, classmates of Rod and Jimmy, arrive at the cave. Their arrival is followed by several others until the group has grown into a large colony that has outgrown the small cave. Rod is the unspoken leader of this group, but a confrontation with a couple of boys protesting Rod's leadership causes one of the college boys to suggest an elected leader should be chosen. At a meeting that night, Rod loses an election to this college boy.

The new mayor, Grant Cowper, appoints committees to begin making laws and addressing the needs of the colony. Fairly quickly, the younger students begin to notice that Cowper is making laws that benefit himself and his fellow college friends, but making rules against the younger students. When Rod points this out, Grant agrees and asks Rod to join his government. Rod reluctantly agrees and takes on the role of leader of the guard. Soon after, Rod talks Grant into allowing him to search the areas around their settlement for a more defensible location.

Rod and another boy, Roy, explore the areas outside the settlement and find a dead sea where the beach is covered in bones. Roy and Rod also find a network of caves that would be large enough to the whole colony and easy to defend from predators. However, when Roy and Rod return from their journey they discover the settlement has grown to include many houses and that no one wants to move. A strange migration begins soon after that highlights the vulnerability of the settlement. Grant dies during this migration, making Rod leader once more. Despite new support for a move to the caves, Rod decides to honor Grant's memory and remain at the settlement.

Under Rod's direction, the settlement is surrounded by a large fence that will protect the settlement much better during the migration season. Rod also directs the building of a farm and an iron works, bringing progress to the settlement. They also begin having dances in the evenings. One day, however, the people from Earth reappear, eager to bring the students home. Despite their outspoken desire to remain only a few days before, all of the colonist but Rod elects to return to Earth. Rod is alone when his teacher, Deacon Matson, and his sister come to see him. They tell Rod that his father has been cured and returned home and that he could fulfill his dream of an Outlander profession if he would come home. Rod decides to go home and becomes the captain of a team of pioneers prepared to settle a new world.