Curt and Robin Faraldi are adoptive siblings. Robin is from another city, but Curt is from another planet. As a teen, despondent Curt vows to find his real home. His wishes come true when the Faraldi house begins exchanging with a building on Curt's original world. Trapped between planets, Curt and Robin must find a path to normalcy or the final exchange will be their life for the afterlife.
The planetary exchange begins when Curt's mother finds cocaine in his room, and Curt ostracizes himself from his family. Because Curt feels he lives in a prison, his parents' property begins exchanging with a prison on Curt's original world. The exchange begins with common objects in the Faraldi house turning into alien equivalents: the lawnmower is an anti-gravity sled , the hair dryer blows snow, the aquarium becomes an alien moat with huge sea creatures.
Then the aliens arrive. Two people who look like Curt come to the Faraldis' front door only to flee. But with this contact comes a terrible portent: the clothes worn by Robin and the alien woman exchange. When the family dog, Hodgepodge, fights with an alien animal and parts of their brains exchange, it's clear that the humans will be next.
The Faraldis soon learn the nature of the ongoing planetary exchanging: Any Terran room leads to a vast alien corridor, but the Terran hallway leads to alien chambers full of dangerous science.
Trying to escape, the Faraldis enter their car only to find themselves flying across the alien land, where they crash into the moat. Robin is swallowed by a huge fish, but Hodgepodge saves her. Ned and Christine are captured by the aliens and imprisoned for trespassing. Though the teens and the dog escape, they face a new dilemma: not only do they have to find a way home, they have to rescue their parents first. In their attempts to save their parents, the teens enter a series of visionary escapades filled with danger and warmth and humor, from battling a horde of alien plushies to venturing in an interstellar spaceship that can barely make it down the hall.
Just as Curt is the cause of the exchanging, Robin is the emotional core of their success. When she begins exchanging body parts with the alien woman, Robin faces the loss of her identity, and her life.