Caliban's War is the second book in James S.A. Corey's The Expanse series. The book takes place sometime after the events of the first book in the series, Leviathan Wakes. It follows the crew of the Rocinante, a former Martian Navy gunship that has been stolen to be used in the human navy, as they become embroiled in a war between Earth, Mars, and the colony of Ganymede, which is ruled by neither the Martians nor the humans.
The story starts with the discovery of a strange alien structure on Venus, which very quickly becomes the focus of a sort of arms race between Earth, Mars, and the Outer Planets Alliance (OPA), all of whom want to claim it for themselves. At the same time, the situation on Ganymede is rapidly deteriorating. Children are disappearing and tensions between the settler and the United Nations (UN) peacekeepers stationed there are rising. When a contingent from the Martian military arrives on Ganymede to secure the alien structure, the situation spirals out of control and an all-out war ignites between the three major powers.
In the meantime, the crew of the ship Rocinante is tasked by the government of the United Nations with transporting a team of scientists to Ganymede to study the alien structure and uncover the secrets behind it. Along the way, they encounter a series of obstacles and challenges, including sabotage, political intrigue, and violent conflicts. They also must contend with aggression from the three major superpowers. As the crew of the Rocinate and the scientists they are carrying learn more about the mysteries of the alien structure, they start to realize that the war on Ganymede is only the start of the bad things that will come; there is something even more sinister and evil at going on. Something which threatens all three powers.
The book ends with a final massive showdown between the crew of Rocinate, the scientists they are carrying, the UN peacekeepers, the Martian military, and the alien race. And after working hard, the crew is able to destroy the alien structure, thus preventing the aliens from enacting their plans and destroying life as the humans and Martians know it. But both sides have to sacrifice quite a bit to destroy the structure; both sides lose people. In the end, the characters in the novel reflect on what has just happened. They consider the cost of war and the importance of working together to protect humanity and the future of their race and the solar system–things that the alien race had threatened to do.
