Ryland’s last and most impactful memory finally flashes back. He recalls Stratt reeling in the wake of the death of the primary crew. Rather than call the tertiary backup with coma-resistance, Stratt decides that Earth’s best chance is if they send Ryland Grace. He refused, claiming first that he could do more good on earth than as an astronaut, and then under her questions, breaking and admitting he was terrified to go to space. She ignores his protestations and drugs him into complete sedation, also administering an amnesiac so he will forget that he was unwilling. Ryland remembers promising Stratt that he will defect upon remembering, but she is confident that he will do the right thing.
Onboard the Hail Mary, Ryland is devastated to realise that she is right. More disappointment in the lab: Rocky and Ryland see that the Taumeaoba samples that they worked so hard to retrieve cannot survive the simulated environments of Erid and Sol. The scientist and the engineer do not give up, and decide to systematically test what caused the death of the Taumoeba and discover that it is nitrogen.
The world of Erid has almost no biological sickness and Rocky tells Ryland that the Eridian immune system is mostly based on heat. Ryland explains the theory of biological evolution and superbug breeding to Rocky and together they decide to breed nitrogen-resistant Taumeoba.
Ryland devises a scientific protocol and Rocky scurries to build the breeding farms out of xenonite.
Eventually they have a strain of Taumoeba that will survive both Earth and Erid. Their mission done, the pair prepare to separate after their long mission together and exchange gifts. Erid will receive a laptop full of earth science and earth will receive xenonite. Ryland finds celebratory vodka and Rocky dresses up in an outfit that sounds good to Eridian senses.
The experiment was not entirely perfect, and some of the astrophage fuel bays could not be cleaned of the predator, and had to be jettisoned, as any direct exposure to the Tauomoeba would leave the ships powerless in space.
Soon, the Hail Mary is fueled up, Rocky boards his own ship again, and both friends realise that due to life span and time dilation, they will never meet again.
Ryland sulks about his dwindling food supply and the loss of his friend, periodically turning on his ship’s equipment to look for the last remnants of his friend in space. On one of these excursions, he realises to his horror that the tiny Beetle engines are dead, all the fuel eaten by the Taumoeba.
He tears apart the ship looking for a leak but finds none. In fact, the main breeder tanks used to develop the nitrogen-resistant strain are all infected. An experiment confirms his fears. Epoxy and plastic are effective barriers for Taumoeba, but due to the generations of breeding, the new Taumoeba can “hide” in the materials of the container to escape death by nitrogen. Ryland reflects on the limitations of playing fast and loose with science. His ship was saved in time, but Rocky’s ship, with only xenonite onboard, is stranded in space. Rylan is left with 2 options. Fly home a hero, or send home the beetles with earth’s salvation, and rescue Rocky and Erid. After being given a second chance at survival, he once again faces the inevitability of death by starvation in space.
He hunts down the blip-A ship, and because no tracking systems can detect him, throws himself at the hull, hammering for Rocky’s acute senses. The friends are delighted to reunite and explain the discovery to each other. Ryland secure’s Rocky’s ship, and Rocky promises that his people will build him a new ship, but Ryland reminds him that he will die and Eridian food is toxic to him.
Rocky comes up with a novel idea, eating the Taumoeba, a biological alien species.
A flash forward. Rylan has been living on Erid for years. The heavy gravity warped his body and malnutrition weakened him before Eridian scientists cloned his own tissue for him to eat. He lives in a life support system and teaches Eridian children, reuniting with his job as a school teacher.
Rocky visits him and is bursting with good news. The scientists report that Earth’s sun is once again shining with full luminance. Ryland cries. The two friends wonder about life on other planets, but remain on Erid, knowing Earth is safe.