When he wakes up at the beginning of the book the hero doesn't have a clue as to who he is or where he is. All he knows is that he's hooked up to a lot of tubes and a computer voice is asking him what 2 + 2 is. When he's finally able to answer that question, the computer allows him to progress in his recovery. As he gets stronger physically, he begins to have flashes of memories (including his name, Ryland Grace). Oh and he discovers there are two people long dead in the beds next to his and that he's in a ship in space. Ryland is all alone and millions of miles away from home. As he explores his current circumstances he is piecing together the moments in his past that have brought him to this point. First among those memories (beyond who he is) is the knowledge that Earth was facing an extinction level event and his mission is to discover a way to save the planet and humanity.
As did the protagonists in Andy Weir's earlier books, The Martian and Artemis, so too must Ryland use every bit of know-how he has to science his way out of the disastrous problems he faces. When Ryland discovers he is not the only one searching for a solution, he realizes that his interstellar adventures may just be beginning. Weir once again seamlessly blends an action plot with hard science and humor to perfect effect.