Nemo, a spirited young girl, lives a happy and secluded life in a lighthouse with her doting father, Peter, who teaches her about the sea and tells her fantastical stories of a dream world called Slumberland, which he claims to have visited with his old partner, Flip. One day, Peter mysteriously disappears at sea during a storm. With no other family, Nemo is sent to live with her estranged uncle, Philip, in a bustling city. Philip is a quiet, reserved man who runs a doorknob business, and Nemo struggles to adapt to her new, mundane life, missing her father deeply.
One night, Nemo discovers a secret map to Slumberland hidden in her father's old possessions. She then encounters Flip, a flamboyant, horned outlaw from her father's stories, who appears in her dreams. Flip explains that he and Peter were once partners in Slumberland, notorious for stealing magical pearls that grant wishes. Flip believes Peter hid a final pearl somewhere in Slumberland, which could reunite Nemo with her father. Nemo agrees to help Flip find it, hoping to use its power to bring Peter back.
Their journey through Slumberland is an adventurous romp through diverse dreamscapes, from a shimmering city of glass to a chaotic realm of alarm clocks. They encounter various dream creatures and are pursued by Agent Green, a rigid dream enforcer who seeks to capture Flip for his past crimes and prevent any disturbance to the natural order of dreams. As they search, Nemo learns more about her father's past with Flip and the true nature of Slumberland. She also discovers that Flip isn't just a figment of her imagination; he is a 'somnum-naut,' a being born from a dreamer's subconscious, and Peter was his dreamer.
Nemo eventually realizes that the pearl doesn't grant wishes in the literal sense; it allows the user to dream a specific dream into reality. She discovers that Peter's final wish, hidden within a specific memory of a dance in the lighthouse, was not for himself but for Nemo to find happiness and courage without him. She learns that the real pearl is not a physical object but a profound understanding of herself and her connection to her father. With this realization, she confronts her grief and understands that her father is gone but his love remains. She uses the pearl's power to have one last, beautiful dream with her father, allowing herself to say goodbye. She also helps Flip, whose existence was tied to Peter's dreams, to find his own independent identity. Nemo ultimately accepts her new life with Uncle Philip, enriching his world with her imagination and spirit, carrying her father's memory not as a burden, but as a guiding light.
