Project Hail Mary

In 2032, Ryland Grace awakens from an induced coma on an interstellar spacecraft. Initially erratic and amnesic, Grace learns that he is the sole survivor of the three-person crew on a spaceship light-years from Earth, which embarked on its journey to the Tau Ceti system over ten years earlier.

Throughout various flashbacks, Grace remembers that he is an American middle school science teacher and former molecular biologist. Years ago, scientists observe an infrared line, named the "Petrova Line", forming from the Sun to Venus. They discover a microorganism, termed "astrophage", proliferating on the Sun's surface, causing it to dim, and predict it will cause catastrophic global cooling within 30 years; Head of the Petrova Taskforce, Eva Stratt, recruits Grace and other scientists to study it.

Grace discovers that astrophages are single-celled organisms impenetrable to electromagnetic radiation. The astrophage breeds on Venus, using the planet's atmospheric carbon dioxide and energy and hydrogen from the Sun. The Petrova line is created by astrophage propulsive emissions as they migrate between the Sun and Venus. Their emissions can be used to construct an incredibly efficient spacecraft engine. The astrophages have infected other stars within Earth's solar neighborhood; Stratt discloses Project Hail Mary, an international effort to send a crew to Tau Ceti, the only undimmed nearby star, to investigate. It is a suicide mission; the Hail Mary spacecraft can carry only enough astrophage fuel for a one-way trip, but the crew's research will be sent back to Earth via probes.

In the present, as Grace approaches Tau Ceti, an alien spacecraft docks with the Hail Mary. It is made of a solid material composed mainly of xenon, which Grace dubs "xenonite". The ship's pilot is a rock-like, five-legged, crab-like alien from a planet in the 40 Eridani A system; Grace calls the alien "Rocky". He infers that Eridians "see" via echolocation, and creates a machine translation system to interpret Rocky's musical chords-like speech. Rocky is a mechanical engineer and also the sole survivor of a mission to stop the astrophage. Neither can survive in the other's atmosphere, so Rocky enters Hail Mary using a small, pressurized ball of xenonite as a space suit. As they learn to work together, the two develop a genuine friendship.

Following Tau Ceti's Petrova line, Grace and Rocky discover that the planet Tau Ceti e, which Grace names "Adrian", harbors an organism ("Taumoeba") that preys on the astrophages, controlling their population. After learning that Grace cannot return home, Rocky offers enough astrophage to refuel the Hail Mary.

Grace continues to remember more and flash back to the origins of his mission: when an astrophage explosion kills the original crew's scientist and his backup, with no time to train a replacement, Stratt asks Grace to take his place. When he refuses on the basis that he is untrained, Stratt has him drugged and put aboard the ship. Grace struggles to come to terms that he was not brave enough to volunteer for the journey.

While gathering the organism from Adrian's upper atmosphere, a fuel leak causes an uncontrollable spin that renders Grace unconscious. Rocky breaks his spacesuit and saves Grace, but is severely injured. While Rocky recovers in hibernation, Grace learns how to selectively breed Taumoeba to survive in Venus's atmosphere. Rocky revives, and the two friends depart for their home planets.

En route to Earth, Grace discovers that the organisms have evolved to pass through their xenonite containers and are eating Hail Mary's astrophage fuel. Grace repairs the problem on his ship but realizes that Rocky's ship is made entirely of xenonite, which would allow the Taumoeba to consume all his fuel, stranding him and dooming his world. Grace chooses to save Rocky and the Eridians rather than return home, sending the Taumoeba and his research to Earth via the probes.

Stratt receives the probes, allowing humanity to stop the astrophage. On the Eridians' planet, Grace lives in an artificial biodome. Rocky tells him that Eridian scientists have finished preparing Hail Mary for its return to Earth, and Grace considers whether to return. While contemplating the news, Grace begins another day of teaching science to Eridian children.