Wikipedia
When the Ares III mission on Mars is forced to abort and leave in a hurry,
Mark Watney is left behind, presumed dead. He is actually alive, and manages
to survive on Mars by cultivating indoors potatoes that were part of the
special Thanksgiving meal. Meanwhile NASA discoveres activity on the Ares III
site and deduces Watney is actually alive. Watney recovers the Pathfinder
probe and uses it to communicate with Earth. NASA attempts to send him food
supplies so that he could survive until the arrival of the Ares IV mission 3
years later. An accident destroys Watney's crop and the supply launch by NASA
fails. Then someone finds out that the rest of Watney's crew, flying back to
Earth on the Hermes could actually slingshot around Earth and return to Mars
to pick him up, provided that he would travel 3000 km to the Ares IV site
where the return rocket is already waiting for the mission. Although not
authorized by NASA, the crew immediately decides to fly back to Mars.
Meanwhile, Watney manages to pack all the neede supplies in a rover and travel
to the site, take all the unnecessary parts off the rocket to reach as high an
orbit as possible. This is still not enough, so the Hermes has to expell most
of its atmosphere, acting as a retrorocket, to slow down and successfully
catch Watney.