While 12-year-old Mae's (Cailee Fleming) father (John Krasinski) awaits surgery, she stays with her grandmother in an apartment in Brooklyn. Worried about her father, Mae gets a distraction when she believes she sees a little girl going into an apartment on the third floor of the same building. She knocks at the door, where she meets Cal (Ryan Reynolds), who lives with a number of imaginary friends (IFs) who have been forgotten or discarded by their children.

Although most people can't see them, Cal discovers that Mae can, so he reluctantly accepts her offer to help them get new friends.