The sun has suddenly begun to dim at an astonishingly fast rate, so much so that people will soon find themselves starving in a catastrophic ice age unless scientists determine what is happening and what can be done about it. The dimming seems to be related to an arc of infrared light between the sun and Venus called the Petrova line after the astronomer who discovered it. A Japanese probe returns a sample of what's in it: single-celled, alien microbes dubbed Astrophage, that consume the sun's energy and emit infrared light when they move at near light speed to Venus, where they consume carbon dioxide to reproduce. It turns out that the sun is not the only local star whose output is dimming. The affected stars all surround a particular star, Tau Ceti, which is 12 light-years away and should also be dimming but for some reason is not. The world's space agencies collaborate on a mission to send a team of three astronauts to Tau Ceti to find out why it's not affected. Russian engineers devise a "spin drive" which harnesses the energy in Astrophage to power a small spaceship, the Hail Mary, on a three-year journey to Tau Ceti at relativistic speeds. When the curtain goes up, we see the ship on its final approach and zoom inside the hull where a lone astronaut, Ryland Grace, is beginning to come out of a medically induced coma with the aid of a computerized nursing apparatus. The coma was necessary to avoid the psychological complications of housing three people in a cramped space for such a long time. Unfortunately, Grace is the only survivor, his two crew mates are long dead husks. To make matters worse Grace, the ship's science specialist and former junior high school science teacher, has amnesia when he wakes up. He does not know who he is, much less where he is and why, but in a series of flashbacks, he gradually remembers the events surrounding the discovery of Astrophage and his part in it.