12-year-old Bea moves into her grandmother Margaret's apartment in New York while her father waits for heart surgery in the same hospital where her mother died years earlier. One night, Bea sees an unfamiliar creature and follows it back to her grandmother's building. The next day, she sees it again, accompanied by a man. She follows them to a nearby house where the man, Calvin, retrieves a large, furry, purple creature named Blue. Bea also meets the other creature, a butterfly-like being named Blossom, and faints. Bea awakens in Cal's apartment, where she learns that he has been working with imaginary friends (IFs) to place them with new children as their original creators have grown up and forgotten them, and they will soon disappear. Initially reluctant, she decides to help Cal.
