Project Hail Mary

The story follows two storylines, each told chronologically, starting with the story on board the spacecraft Hail Mary where Ryland Grace regains his memory in bursts. This story is frequently intercut with flashbacks revealing earlier events leading up to the launch of the Hail Mary.

In the near future, a global dimming event is observed, coinciding with the formation of a bright line from the Sun to Venus. The exponential rate of dimming is calculated to result in a catastrophic ice age within 30 years. A space probe is used to discover that the line appears to contain alien microbes. The world's governments cooperate, giving former European Space Agency (ESA) administrator Eva Stratt unilateral authority and legal immunity to solve the problem.

Stratt nominates Ryland Grace, a teacher and former molecular biologist, as the first person to study a sample of the organism recovered from Venus, as she views him as expendable and his research as a molecular biologist had predicted the type of life the sample is suspected to be. He discovers that the single-celled organism consumes all forms of electromagnetic radiation and uses radiant energy to move. Since the organism consumes energy from the Sun, Grace names it "Astrophage" (Greek for "star eater"). Grace subsequently discovers that Astrophage reproduces by feeding on the heat of the Sun and carbon dioxide on Venus. Other scientists discover that Astrophage employs mass–energy conversion via neutrinos, and can be mass-bred for use as rocket fuel.

Astronomy data reveals that Astrophage has also infected and dimmed nearby stars, but one star, Tau Ceti, has unexpectedly resisted Astrophage infection. An Astrophage-fueled starship, the Hail Mary, is developed to travel to Tau Ceti to obtain knowledge of Astrophage resistance, and to return the findings to Earth with unmanned mini-ships ("beetles", with each named after their respective Beatle counterpart). Hail Mary can only be fueled and supplied for a one-way trip, making it a suicide mission.

Stratt orders the release of methane from Antarctica to reduce global cooling, but the threat of famine and wars over food on Earth remains for the 26 years that the Hail Mary mission is expected to take.

Since the crew of three astronauts will travel under a coma to avoid psychiatric issues, potential astronauts are restricted to people with coma-resistant genes, including Grace. Stratt tasks Grace to train the science experts for the mission, but the science experts are killed in an Astrophage-induced explosion shortly before launch. With no time to train a replacement, Stratt asks Grace to join the mission. When he declines, she forces him. Grace threatens sabotage, so a sedative is administered to him before launch, with a temporary amnesia-inducing drug set to be administered shortly before he awakens.

Ryland Grace emerges from his coma with no memory of his identity or situation. As the mission proceeds, Grace deduces his situation while his memory returns gradually. Grace finds that his crew members have died en route, and gives them a space burial.

Hail Mary soon reaches Tau Ceti, and is approached by an alien starship, which Grace names "Blip-A". The other ship indicates that its home system in 40 Eridani is also plagued by Astrophage infection. Blip-A docks with Hail Mary, and the two ships establish first contact. Grace develops a system to communicate with the spider-like alien, whom Grace names "Rocky" due to its stone-like exoskeleton. Rocky, a skilled engineer, has been in the Tau Ceti system for 46 years as the last survivor of his crew; Grace deduces that the others died from space radiation, an unknown concept to Eridians.

Also unfamiliar with the concept of relativity, the Eridians greatly overestimated the amount of fuel needed for a round-trip to Tau Ceti, and the Blip-A has enough Astrophage fuel for both ships to return to their planets. Grace realizes that what was once a suicide mission now may end with him returning to Earth. He fantasizes about confronting Stratt for coercing him to go.

Grace and Rocky discover the Astrophage's home planet in orbit around Tau Ceti, which Rocky names after his mate. Grace deciding on "Adrian" as the English translation, after Adrian Pennino, the wife of the fictional Rocky Balboa (i.e., the planet is now named for this other "Rocky's mate"). The planet's atmosphere is found to contain a natural predator of Astrophage. While collecting a sample from Adrian, a hull breach occurs and Grace and Rocky both risk their lives to save each other. Grace is severely burned when saving Rocky, who sustained severe injuries but eventually recovers. From the sample, the Astrophage predator is found to be a microbe which Grace names "Taumoeba".

After a Taumoeba outbreak on the Hail Mary, where all Astrophage in the ship's fuel tanks are consumed by the Taumoeba, Grace and Rocky are captured in a decaying orbit over Adrian. They escape by mounting the beetles on the ship's hull and using them as mini thrusters. Grace and Rocky then use selective breeding to produce Taumoeba that can survive in both their home solar systems.

Rocky refuels the Hail Mary, and they part ways. En route, Grace discovers that he has accidentally bred into the Taumoeba the ability to permeate and escape their containers. Although Grace fixes the problem on Hail Mary, he realizes that the Blip-A is made of the same material as those containers, and that the Taumoeba would have consumed its fuel, leaving Rocky stranded.

Due to a limited food supply, Grace is forced to choose between personally returning to Earth as a hero but dooming the Eridians, or saving Rocky and the Eridians but then starving to death on Erid (where the environment can't produce food for him). Grace chooses the latter, sending the beetles back to Earth with the Taumoeba, while he locates the fuel-less Blip-A and reunites with Rocky three months after their departure. Rocky is overjoyed and points out the possibility that Grace could consume Taumoeba, giving him a chance at survival.

They return to Erid, fixing the Eridians' Astrophage problem. Using the digital archive of human knowledge given to Grace by Stratt, the Eridians build a system to enable Grace to live on Erid, where he becomes a teacher of Eridian young. Around 16 years later (the amount of time it takes for light from our sun to travel to Erid), Rocky tells Grace that the Astrophage infection around the Sun has abated too, meaning Grace's mission was a success and that Earth's scientists figured out a solution after the return of the beetles. Knowing that humanity has survived on Earth, Grace considers returning to Earth, with the Hail Mary still operational and in orbit around Erid but decides against it since he has friends on Erid (especially Rocky) and he finds his new teaching career rewarding.