Project Hail Mary

In the near future, middle school teacher and former molecular biologist Ryland Grace wakes up from an induced coma on the interstellar spacecraft Hail Mary, suffering from amnesia. Grace learns that he is the sole survivor of a three-person crew that was traveling towards the Tau Ceti system, 11.9 light-years from Earth.

In a series of flashbacks, Grace recalls that scientists discovered a "Petrova line" of infrared light stretching from the Sun to Venus; an alien microbe in the line, dubbed "astrophage", was proliferating on the Sun's surface, dimming the Sun at a rate that would cause catastrophic global cooling within thirty years. The head of the Petrova Taskforce, Eva Stratt, recruited Grace to study it due to his background in speculative astrobiology. Grace experimented on a sample and discovered that astrophage was composed of unicellular organisms that absorb electromagnetic radiation from the Sun and expel it for propulsion.

Grace joined Stratt's Project Hail Mary, an international "long-shot" mission to send a crewed spacecraft to Tau Ceti, the only star in Earth's solar neighborhood not occluded by astrophage. The flight to Tau Ceti would be a suicide mission, as the ship could only carry enough fuel for a one-way trip; any findings would have to return to Earth in smaller space probes.

In the present, Grace encounters an alien spacecraft that docks with the Hail Mary. It is made of a solid form of xenon, which Grace dubs "xenonite". The ship's occupant is a rock-like, five-legged alien from Erid, a planet in the 40 Eridani A system. Grace names the alien "Rocky", deduces that Eridians "see" via echolocation, and creates a machine translation system to interpret Rocky's musical language into English. Rocky is a mechanical engineer and the sole survivor of the Eridians' mission to save their own star from astrophage infection. Grace and Rocky agree to work together and become close friends. As neither can survive in the other's atmosphere, Rocky works aboard the Hail Mary inside a pressurized ball of xenonite.

Grace and Rocky study the Petrova line of the planet Tau Ceti e, which Grace names "Adrian", and discovers that the line hosts an entire microbial biosphere. Rocky theorizes that microbial organisms in Adrian's atmosphere are consuming the astrophage, keeping the population in check. Learning that Grace cannot return home, Rocky offers to share some of his own ship's astrophage fuel with the Hail Mary. Grace remembers that an astrophage explosion killed the original crew's science officers. With no time to train replacements, Stratt asked Grace to join the mission.

During a risky maneuver to gather the astrophage predator from Adrian's atmosphere, a fuel leak causes the Hail Mary to spin uncontrollably, rendering Grace unconscious. Rocky breaks out of his ball to save Grace but is severely injured. While Rocky recovers, Grace studies the captured astrophage predator, dubbing it "taumoeba" and selectively breeding it to survive the nitrogen in Venus's atmosphere. Rocky revives, and the two celebrate their success; Grace recalls that he fearfully refused to join the Hail Mary crew, but Stratt had him drugged and put aboard the ship anyway, believing he was the only one capable of completing the mission.

Rocky and Grace say their goodbyes and begin their journeys back home. Shortly thereafter, Grace discovers that the taumoeba has evolved to escape its xenonite breeding tanks and is starting to consume Hail Mary's astrophage fuel. Grace stops the taumoeba leak, but realizes that Rocky's ship, being entirely made of xenonite, will have no way to contain the taumoeba; it will eat all Rocky's fuel, stranding him and dooming Erid. Grace decides to use his remaining fuel to rescue Rocky and the Eridians rather than return home, and sends his video logs and taumoeba samples to Earth via the probes.

Stratt and her team receive the probes and use the taumoeba to cure the Sun's astrophage infection. Meanwhile, on Erid, Grace lives in an Earth-like biodome the Eridians have constructed for him. Rocky informs him that Eridian scientists can prepare the Hail Mary for the return voyage to Earth. Grace ponders the news before beginning another day of teaching science to eager Eridian children.