Before losing his memories, Grace had been recruited for a last-ditch mission known as Project Hail Mary. His spaceship is headed toward a star called Tau Ceti after astronomers detected evidence of an alien civilization there. Grace is tasked with making first contact and figuring out if the aliens can help save humanity. If not, his backup plan is to scour the alien planet for resources or technology that could cure the blight.

As Grace adjusts to life alone on the Hail Mary, he befriends an alien lifeform he names Rocky. Rocky is a macroscopic symbiote that lives in the ship’s astrophage, an organism that feeds on radiation. The unlikely duo communicate through gestures, drawings and a shared love of science. Rocky becomes Grace’s research assistant, helping him conduct experiments to learn more about the alien species and their biology.

When the Hail Mary finally arrives at Tau Ceti, Grace takes a smaller ship to the alien planet’s surface. He makes contact with the aliens, who reveal they already know about Earth’s crisis. The aliens explain that the blight plaguing Earth is caused by their own microscopic symbiotic lifeforms run amok. Working closely with Rocky, Grace is able to develop a way to stop the rampant astrophage and save Earth’s plants and ecosystems.

With this cure in hand, Grace begins the long journey back to Earth. But cosmic rays damage the Hail Mary along the way, causing life support systems to start failing. With oxygen running out, Grace decides on a risky maneuver to use the ship’s gravitational slingshot to arrive home faster. After a harrowing ordeal, he safely crash-lands the Hail Mary in the ocean near Earth.

Now a hero, Grace reunites with his friends and colleagues from before the mission. The cure is quickly distributed and Earth begins recovering from the brink of catastrophe. Grace continues his research and advocacy for science education, with Rocky coming to live with him on Earth. Having survived against all odds and saved humanity from extinction, Ryland Grace embraces his new lease on life with those he loves.