Did you like The Martian? If so, then you'll like Project Hail Mary. It has the same nerdy smart-ass narrative voice, the same lone-guy-solving-science-problems plot, and the same concern for accurate hard sci-fi. I really think Weir should have made Project Hail Mary the second book in a series: The Martian's astronaut Mark Watney would make more sense in every way than the junior high-school teacher Ryland Grace.
The scientific challenge is far more consequential than the one in The Martian. The fate of all humanity is at stake. An alien organism is gobbling up energy from the Sun. An international research mission heads to the nearest star that appears to have evaded the organism. Only our intrepid narrator survives the trip, and he is just recovering his memory of the mission's purpose when he encounters an alien spaceship apparently on a similar expedition.
There are three major storylines:
- Research into the existence and properties of the alien organism Astrophage, with the goal of finding a "cure" to eliminate it from our solar system
- Learning to communicate and collaborate with the other intelligent alien species
- Designing the spaceship and planning the mission
I've listed the storylines in order of plausibility, or perhaps I should say ease of suspending disbelief. The success rate for Grace's jerry-rigged experiments is too high, and I felt that he ignored the Astrophage problem for too long while learning to communicate with the alien, but these nits didn't undermine my enjoyment of the story. On the other hand, the flashbacks to mission planning on Earth are entirely unbelievable, starting with the teacher being given a central role in the project over literally every other scientist and astronaut in the world.
Project Hail Mary would be far better if Weir found a more realistic way to convey the necessary backstory.