Project Hail Mary

Ryland Grace awakens from an induced coma on an interstellar spacecraft. Initially erratic with retrograde amnesia, Grace discovers that he is the sole survivor of the three-person crew on board, and that he is light-years away from Earth in a distant solar system.

Grace slowly remembers that he is an American middle-school science teacher and former molecular biologist. In the past, scientists observe the Sun dimming, and the formation of an infrared line from the Sun to Venus called the Petrova line. The dimming, caused by a microorganism known as an astrophage exponentially increasing its population on the Sun's surface, will result in a catastrophic global cooling of the Earth within 30 years. Government agent Eva Stratt recruits Grace and other scientists around the world to study astrophages.

Grace discovers that astrophages are single-celled organisms impenetrable to any form of electromagnetic radiation. Further work reveals that astrophages breed on Venus, feeding on the planet's carbon-dioxide atmosphere and energy from the Sun, and that they have infected other stars within Earth's solar neighborhood. Researchers have also found that astrophages can be used to construct an incredibly efficient rocket engine, although their extremely powerful energy release makes them dangerous to use. Stratt discloses Project Hail Mary, an international effort to send a crew to Tau Ceti, the only undimmed nearby star, to learn why that star is not dimming. It is a suicide mission: the Hail Mary spacecraft can only carry enough astrophage fuel for a one-way trip, but will carry probes to send the crew's findings back to Earth.

In the present, as Grace approaches Tau Ceti, he sees an alien spacecraft. The spacecraft, which docks with Hail Mary, is made of a solid form of xenon that Grace dubs "xenonite". The pilot of the other ship is a rock-like, five-legged alien from 40 Eridani, whom Grace names "Rocky". Grace deduces that Rocky sees via echolocation and creates a machine translation system for Rocky's musical speech. Rocky explains that he is a mechanic and also the sole survivor of a mission by his species, the Eridians, to stop the astrophage. Neither can survive in the other's atmosphere, so Rocky enters Hail Mary using a small, pressurized ball as a spacesuit.

Following Tau Ceti's Petrova line, Grace and Rocky find that the planet Tau Ceti e – which they name "Adrian" after Rocky's mate – harbors an organism that preys on astrophage, controlling their population. After learning that Grace cannot return home, Rocky offers enough astrophage to refuel Hail Mary. While gathering the organism from the upper atmosphere of Adrian, 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 finds a way to breed the astrophage-consuming organism, which he names "taumoeba." Rocky revives, and he and Grace part as friends to their home planets.

In the past, Grace meets Hail Mary's commander, engineer, and scientist, and their backups. While doing an experiment with astrophage, an accident kills the scientist and their backup three days before launch. With no time to train a replacement, Stratt asks Grace to take the place of the deceased scientists. When Grace refuses, Stratt has him drugged and forcibly put on Hail Mary.

During the trip to Earth, Grace discovers that the taumoebae have evolved the ability to pass through their xenonite containers and are eating Hail Mary's astrophage fuel. Grace fixes the problem on his ship but realizes that Rocky's entire ship is made of xenonite, allowing the taumoeba to consume all of his fuel. Left with the choice of returning to Earth or saving Rocky and the Eridians, Grace chooses the latter, sending the taumoeba and his research to Earth via the probes.

The probes deliver to Stratt what humanity needs to stop astrophages from absorbing the Sun's energy output. The Eridians create a biodome on their planet for Grace to live in. Rocky tells him that Eridian scientists have finished preparing Hail Mary to return to Earth. While contemplating the news, Grace begins another day of teaching science to Eridian children.