Marty and the Magic Bag tells the story of a boy's childhood imaginary friend — an alien from the planet Jupiter named Steve Martin McFly Johnson (Marty, for short). After discovering a magic bag that opens a portal allowing Marty's soul to inhabit the boy's body, Marty and the boy's brother spend their childhood going on adventures together in their imagination. When the boy grows up, he remembers Marty's sacred promise to take his brother to Jupiter as a birthday present for his 40th birthday. Rediscovering the power of imagination, the boy — now an adult — must figure out a way to make the spaceship journey happen. With the help of his own kids, the boy builds a makeshift spaceship and devises a plan to fulfill Marty's promise. In the process of doing so, the boy rediscovers Marty's message: Imagination is real.