Astrophage, an unintelligent single-celled organism, is consuming the Sun's energy along a trajectory known as the "Petrova Line". It is projected to cause a 10% drop in solar output, leading to global crop failures and the death of 25-50% of humanity within 30 years.
Tau Ceti is the only nearby star unaffected by Astrophage. "Project Hail Mary" is an international mission sending a starship to Tau Ceti to investigate why its star is immune and return findings via an unmanned probe. Due to fuel constraints, the mission is a guaranteed one-way journey.
The journey requires astronauts to spend years in a medically induced coma, which requires a rare genetic resistance to survive. Ryland Grace, a middle-school science teacher and former academic, is the world's leading Astrophage biologist and possesses this genetic trait. Grace is by far the most qualified biologist available within the launch window, but despite his deep commitment to understanding Astrophage, he refuses to go. Other genetically viable candidates exist, but they have less specialised knowledge in Astrophage biology and would require rapid, intensive prep.