Through an act of sabotage, the cannon is fired during the 1893 World's Columbian Exposition in Chicago, with several dozen famous people and leaders of the time on board. A second cannon is prepared to find and recover the people in the first, who are stranded on Mars.

It turns out that Mars had an extensive civilization based on plants. Most monsters the player encounters are so-called "plantimals", such as the Jumping Bean and the Planther. Mars has cities and canals, although the civilization is in ruins, so the player's first tasks are restoring the world power station, and melting enough of the polar caps (with a solar lens) to fill the canals.

Some of the people appear to have gone insane after using a device called the Dream Machine. What in fact happened was that, after massive soil poisoning, the original Martians had gone into a sort of alternate dimension called "dreamspace" to preserve themselves. Those people using the Dream Machine found themselves trapped in dreamspace, while the Martians took over their bodies. A large part of the game is spent visiting various people's nightmares and clearing them up.