Plot
The film opens with Rudger, an invisible entity known as an Imaginary, in the midst of a colorful adventure conjured by his creator, Amanda Shuffleup. Their escapades are interrupted by Amanda's mother, Lizzie, who is exasperated by her daughter's aloof behavior and fixation on a friend who, as far as she can tell, does not exist.
While on a day trip with Lizzie, Amanda and Rudger encounter the menacing Mr. Bunting and his own Imaginary, a sullen girl in black. Revealing himself to be a hunter and devourer of exceptional Imaginaries, he decides Rudger is his next mark. As the two children flee from the man in a panic, Amanda is hit by a car, the accident appearing to be fatal.
Now alone, Rudger begins to disappear, since Imaginaries cease to exist once their creator forgets them. Zinzan, a mysterious cat, persuades the boy to follow him to a sanctuary for Imaginaries at risk of vanishing. The sanctuary is a real-life library, where Imaginaries are sustained by the creative energies of the books within. Among the dozens like himself, Rudger befriends the skeletal Cruncher-of-Bones, Snowflake the Hippo, and Emily, who is a leader among the library's inhabitants.
Over dinner that night, Rudger asks Emily about Mr. Bunting, and she explains that he was once a mortal human man who was so desperate to preserve his Imaginary, he resorted to consuming other Imaginaries, which has sustained the both of them for hundreds of years. At the conclusion of her story, she insists Bunting is nothing but a myth, much to Rudger's frustration.
The following morning, Emily tasks Rudger with his new job, where he'll spend the day with a child in need of pretend playmates. He selects John, one of Amanda's classmates, as his first assignment. At the end of the day, he spots Bunting lurking outside of John's home, and confronts him about Amanda coming to harm. Bunting does not accept the blame and attempts to capture Rudger, who narrowly escapes and takes refuge at Amanda's house. There, he observes Lizzie discovering Amanda's momentos of her late father, which reminds him that the man's death was the reason for his creation. He also learns that Amanda is alive in the hospital, though in critical condition.
After Bunting catches up with Rudger, Emily and Zinzan come to his rescue, but they all find themselves trapped in an imaginary world of Bunting's design. Though the party is able to track down an exit leading to the library, just before they cross the threshold, Bunting shoots Emily with a pretend gun, ending her life. Returning to the library, Rudger is horrified to discover everyone else has already forgotten about Emily.
Resolved to encourage Amanda to live, Rudger journeys to the hospital, but Bunting is already there waiting for him. Amanda is awoken from her coma by Rudger's voice and she engages Bunting in a battle of imagination. Lizzie arrives just as Bunting gains the upper hand by creating an enormous snake that slowly crushes Amanda. Unable to see the snake, Lizzie is initially powerless to stop it, but with Rudger and Amanda's help, she remembers her old Imaginary: a sheepdog named Fridge. After successfully calling Fridge, he incapacitates the snake and rescues Rudger from the vacuum of Bunting's maw. Bunting's Imaginary, apparently moved by Amanda and Rudger's bond, allows herself to be devoured instead. As she disappears, so too does Bunting, defeated for good. Safe at last, Rudger shares a tearful embrace with Lizzie and Fridge.
Sometime after Amanda is discharged from the hospital, Rudger is in the process of disappearing, but affirms that no matter where Amanda's life takes her, he'll always be a part of her. The two of them now ready to brave the future, they embark on one final adventure.
