Red Mars opens after humanity has already colonized Mars. Some guy named Frank Chalmers isn't exactly happy with the current state of Martian society and decides he needs to take control of the situation. Using a festival for cover, he plans an elaborate assassination of a fellow political figure and First Hundred, John Boone. Well, that covers the where, but leaves the who, what, how, and why a little vague, doesn't it?
To answer these, the novel rewinds back to 2026 and the first days of the Ares's maiden voyage. The Ares is an interplanetary spacecraft shipping one hundred colonists on a one-way trip to the rust-ridden planet. The First Hundred begin their journey together pleasantly enough, but as time passes, relationships and loyalties form, breaking the once united group into factions with wildly different goals. Of particular note is a Russian named Arkady who argues that they should form an independent state on Mars, while others argue their loyalty remains with Earth.
Once on Mars, the struggle to survive momentarily reunites these one hundred people as they build Underhill, Mars's first human settlement. But, as is often the case with politics, it's not long before the rhetorical throat-ripping ensues again. The most heated debate this time around centers on the issue of terraforming—that is, whether or not they have the right to transform Mars's landscape to be more like Earth's.
Ann Clayborne becomes the figurehead for the Reds (those who argue against terraforming), while Sax Russell leads the arguments for the Greens (those who are gung-ho for some planetary herbage). Caught in the middle are those like Nadia, who can see the benefits of both sides of the argument. After much back and forth, UNOMA grants permission for the terraforming to begin. Around this time, Hiroko Ai and her groupies disappear into Mars's rocky wilderness, taking Michel Duval, the team's psychiatrist, with them.
Fast-forward to the year 2047. Colonists are coming to Mars in the thousands, seeking a life beyond the confines of an overpopulated Earth. Terraforming is now the norm, and Sax Russell leads the project. Working with Sax, John Boone travels between the Martian colonies playing detective. His goal is to discover who is responsible for a series of sabotage incidents at important terraforming sites.
During his travels, he learns that various transnational corporations—a.k.a. transnats—are preparing to use Mars's vast resources to fill their money bins. He also learns they plan to revise the Mars Treaty and build a giant space elevator to make the interstellar robbery easy and even legal. To stop this and make Mars an independent state free of history's mistakes, John gathers most of the First Hundred under a new MarsFirst banner, hoping to politically combat the greedy transnats.
But we already know how well that works out.
After John's death, Frank revises the Mars Treaty with the help of UNOMA officials. He attempts to maintain a political balance between Mars and Earth. It doesn't go so well. The transnats quickly ship thousands of people to Mars, and in order to maintain the best profit-to-cost ratio, they essentially create slums of abused and downtrodden workers. Despite Frank's best efforts to prevent it, the Martian revolution is afoot.
During the war, Arkady is killed, many Martian cities are bombarded, and the space elevator is launched deep into the solar system. Phobos, Mars's moon, is sent crashing into the planet. The First Hundred become targets of the transnats for their political influence.
Nadia travels the planet rebuilding the Martian infrastructure where able, but she can only do so much in the face of war's vast destruction. Ultimately, she, Maya, Frank, Sax, Ann, and Simon are forced to go into hiding. Luckily for them, the long-lost Michel suddenly returns to acts as a tour guide to Hiroko's refuge
A harrowing journey ensues. A massive aquifer breaks and floods the Martian canyon system that is their only path, and Frank sacrifices himself to save the others from icy death. The rest manage to make it to Hiroko's refuge where they must start again to remake Mars into something better than it has become.