Ryland Grace wakes in a sealed sickbay tended by robotic arms and a flat-voiced computer. He can answer basic questions but remembers nothing of himself. Exploring the ship under an unusually strong gravity, he discovers two desiccated crewmates, a pristine lab, and equipment that lets him measure an acceleration of roughly 15 m/s²—he is not on Earth. As memories return, they center on a global emergency: astronomers found a constant-wavelength infrared "Petrova line" from the Sun toward Venus while the Sun itself grew measurably dimmer.

In flashback, UN director Eva Stratt conscripts Grace—formerly a scientist turned schoolteacher—into analyzing a returned sample from NASA’s ArcLight probe. Inside are heat‑proof specks that store and release incredible energy, propel themselves by emitting light, and hold a constant internal temperature: living organisms that feed on stellar output. Grace names them Astrophage and deduces a lifecycle: they charge at the Sun, follow magnetic lines outward, lock onto Venus by carbon‑dioxide spectral lines, reproduce in its upper atmosphere, and return toward the Sun—the Petrova line. A Russian team then shows Astrophage convert mass to energy and back, making them ideal propulsion. With many stars dimming but Tau Ceti oddly unaffected, a mission is formed to go there, study the exception, and beam home a fix.

In the present, Grace reaches the ship’s control room and learns the mission’s name—Hail Mary—and the crew: Commander Yáo Li‑Jie, Olesya Ilyukhina, and him. He is decelerating at 15 m/s² toward a star that is not the Sun. Ship systems show a vast Astrophage fuel load, "spin drives" for thrust, a centrifuge split for 1 g lab work, and four autonomous data probes called beetles. Flashbacks fill in the project’s brutal pragmatism: coma‑resistant crews selected from 1 in 7,000 to survive long sedation; a black‑panel plan to mass‑breed fuel; and Astrophage slurry lining the hull as radiation shield.

Approaching the target system, Grace activates a Petrovascope and sees a local Petrova line: Tau Ceti is infected too. Then a triangular craft slides into view, flashing at the Petrova wavelength. First contact unfolds by mimicry, then by engineered cooperation. The alien ship station‑keeps within a few hundred meters and delivers a sealed cylinder of a strange metal ("xenonite") containing star maps that mark 40 Eridani. Grace replies with a map tagged with Sol. The alien proposes a docking tube. After careful pressure tests—his air vs. their hot ammonia at ~29 atmospheres—a transparent barrier goes in. On the other side is a five‑limbed engineer Grace nicknames Rocky. Communication grows from models to clocks to a chord‑to‑English audio translator when Grace realizes Eridians perceive through sound, not light.

Both travelers are sole survivors sent because their stars are fading while Tau Ceti is bright; both lost crewmates to hazards they could not fully anticipate. With a portable, cooled pressure sphere, Rocky boards Hail Mary. They share tools, restore spin for 1 g science, and plan to sample the thickest Petrova line near a planet Grace nicknames Adrian. Spectroscopy shows an atmosphere rich in CO2 and methane that shouldn’t persist without replenishment, hinting at biology. They retrieve Astrophage and find equal inbound and outbound populations along the line. Under a microscope is a bustling ecosystem, including an organism that attacks Astrophage. Adrian is likely the origin world; its predator could be the cure.

To catch it, they map the Astrophage breeding layer as a razor‑thin IR cloud at about 91 km altitude. The ship cannot fly through air under thrust without catastrophe, so Grace devises a plan: drag a 10 km xenonite chain with a sealing sampler through the layer while thrusting off‑vertical so exhaust misses it. The risky pass works, but the engine’s heat reflects, melting a fuel‑tank hull. Exposed Astrophage “sees” Adrian and jets toward it, creating uncontrolled thrust and spin. Grace jettisons bays to stop the leak but is pinned, suffocating—until Rocky crosses into the human side, cuts him free, and collapses from exposure.

Grace stabilizes the tumbling ship by extending the centrifuge cables, drags Rocky to the Eridian airlock, and—at great personal cost—repressurizes it with 29‑atm ammonia, suffering severe injuries. He then invents a way to purge soot from Rocky’s burned radiator. Rocky recovers, and together they open the sampler. Inside is the Astrophage‑eating microbe Grace names Taumoeba. They culture it and begin testing survival in Venus‑like and Threeworld‑like conditions, hoping to seed each planet to exterminate Astrophage where they breed. Suddenly the ship goes dark: Taumoeba infiltrated an Astrophage generator, consuming the fuel.

Rocky sterilizes a generator and powers Hail Mary from his sealed Astrophage. Grace retrieves beetle probes on a hazardous EVA and, with Rocky’s modifications, turns three into external thrusters to stop the spin and execute a burn. Flashbacks reveal a catastrophe on Earth: a supply error during an edge‑case test gave DuBois and Shapiro a milligram of Astrophage instead of a nanogram; their lab vaporized. With launch windows closing, Stratt forces the coma‑resistant Grace onto the crew, planning amnesia so he cannot refuse.

Back on station, Taumoeba fails in Venus/Threeworld atmospheres. Grace and Rocky isolate the killer variable: nitrogen. They begin selective breeding for nitrogen tolerance, spinning up for seven days of 1 g lab work. While inching toward survivable percentages, they rendezvous with Rocky’s ship, exchange tech, and plan farewells. The final strains—Taumoeba‑80 and Taumoeba‑82.5—thrive in tailored Venus and Threeworld mixes. Rocky refuels Hail Mary to 2.2 million kilograms from Eridian reserves, and both ships pack redundant Taumoeba farms. They part ways: Rocky races for Erid; Grace turns for Earth, beetles loaded with data and living samples.

Mid‑return, Grace discovers Taumoeba contamination in his fuel supply. Investigations show his evolved strain can permeate xenonite; natural Taumoeba cannot. He rebuilds farms with metal and realizes the horrifying implication: Rocky’s xenonite ship and fuel tanks are vulnerable. When the Blip‑A’s engine signature vanishes, Grace chooses to sacrifice his return. He launches the beetles toward Earth and burns back into Tau Ceti space to find Rocky. Using his spin drives as a giant IR beacon and sweeping with the Petrovascope, he locates the drifting Blip‑A, docks, and rescues an injured Rocky. He explains the xenonite‑permeating evolution; they transfer to Hail Mary and set a course for Erid, planning to sterilize Rocky’s ship in port.

Grace has fuel but no food for the round trip. Rocky proposes Taumoeba as sustenance. It works well enough to keep Grace alive through years of travel. In an epilogue on Erid, Grace, older and finally nourished by Eridian science (synthesized vitamins and lab‑grown human muscle), lives in a human‑habitable dome and teaches Eridian children. Rocky brings news: Sol has returned to full brightness, implying Earth received the beetles and executed a planetary fix. Grace, relieved beyond words, puts off any return for now. Having saved one another’s worlds, the friends settle into a new chapter—scientist and teacher among aliens—while the cosmos remains full of mysteries seeded by life itself.