On a dark winter morning, nine-year-old Petey Myshkin Stephenson is taken by a mysterious, battered yellow trolley he has never seen before to a place called "Otherwise." There he discovers a place in the middle of summer, with a green sky, where birds fly upside down and the sun rises in the west. It is also in the midst of a war. The trolley picks up a young girl named Sharlotta, who has just escaped a battle where her family was abducted by the invaders because they believe the father holds the secret to a Spell that will give them control of the worlds of both Otherwise and Howtiz - the "real" world, where Petey lives. And Petey joins Sharlotta to find and rescue her family.

After numerous adventures involving encounters with two young members of the invading tribe and a cruel prince who captured, and later will threaten to torture Sharlotta's family into confessing the Spell, Petey and Sharlotta sneak into the enemy camp and discover where the girl's family is being held, buried underground beneath a black tent. Despite seemingly insuperable obstacles, the two rescue the captives and, after perilous encounters with enemies and false friends - and with the aid of one of the young "enemies" who turns out to be a powerful ally - Petey, Sharlotta and her family finally manage to escape the camp.

In the meantime, the camp has caught fire as a result of Petey's clever ploys during the family's rescue. The camp burns to the ground in a vast conflagration, and the invaders are forced to return home in a humiliating defeat. The Spell is secured and the world is saved, and Petey returns to Howtiz - but not before Sharlotta gives him a key to remember her by.

But after Petey returns home no one believes his story, and even he is not sure: maybe what he just lived through was just a dream after all. Then he discovers a funny lump in his pocket: Sharlotta's key.