Dr. Aaron Nasmeth, a mild-mannered biologist, and Jason Zaya, a flamboyant engineer, are chosen as the best of the best to take up a life-spanning mission: transport a frozen flight crew and 3000 frozen blastocysts across space and time. The destination: three candidate exoplanets for humankind to explore and colonize.

The faster the ship accelerates, the slower mission control receives its messages. Likewise, for Nasmeth and Zaya, replies from Earth take longer and longer to arrive. The people of Earth closely follow the crew’s progress. Rivaling fandoms emerge for Nasmeth and Zaya.

The two men begin to get on each other’s nerves in the close quarters of the spaceship. Zaya continually leaves pedantic notes for Nasmeth, requesting that he clean the toilet seat and organize materials in a very particular way. Their biggest fight is over a door that connects the workroom to the canteen. Nasmeth wants to keep it closed, Zaya, open. Zaya scratches Nasmeth’s name on the door, as well as an imperative: “Keep door open.” Eventually, Zaya removes the door from its hinges and hides it in a storeroom.

Though Nasmeth is a rule follower, he develops a strong desire to murder Zaya. He records a message for mission control stating his intent; by the time they see it on Earth, they have no means to intervene due to the time dilation. Nevertheless, the President sends a direct order prohibiting Nasmeth from killing Zaya.

On Earth, Nasmeth’s announcement goes viral. An old roommate of Zaya’s speaks up and claims he is a psychopath. Nasmeth’s father reveals that Nasmeth was expelled as an eighth grader for violently retaliating against his bullies with a baseball bat. The two men’s respective factions riot and fight with one another; many civilians die.

On the ship, Nasmeth confronts Zaya with a wrench. The two square off in a zero-gravity battle. Blood floats and spirals around them. Nasmeth hits Zaya right in the temple with his wrench and has the opportunity to finish him off, but can’t. In the hubbub, the ship is thrown slightly off-course, losing years worth of time. Nasmeth records a message for Earth mission control, letting them know he didn’t go through with his murder plot.

When the technicians receive the message on Earth, they notice a shadow moving in the background at the very end of Nasmeth’s video—Zaya was still alive and was waiting for Nasmeth to finish recording, after which he killed Nasmeth.

Many years later, the flight crew and blastocysts have been thawed and the ship reaches its destinations. The first two exoplanets are not suitable for life, but the third one is deemed adequate and the crew settles there.

They receive a message from Earth: from the President, originally sent decades earlier, instructing Nasmeth not to kill Zaya. Many of the crew members bear Zaya’s name, but no one remembers Nasmeth. They check the flight crew history … and Nasmeth is nowhere to be found—Zaya has erased his existence.

But the ship’s new commander recognizes Nasmeth’s name—he has seen it scratched into an odd door in the storeroom. The commander, whose wife is one of Zaya’s grandchildren, scratches out Nasmeth’s name on the door until it is gone.