Born from the creativity of Amanda's luminous mind, Rudger is an imaginary friend that she alone can hear or see. Together, they go on wild adventures in the attic above the bookshop owned by Amanda's mother. The two have pledged to follow three rules: never disappear, always protect one another, and never cry.

One day, a suspicious man named Mr. Bunting knocks on the door, accompanied by an eerie girl that only Rudger and Amanda can see. He has a keen nose for hunting imaginary friends and depriving children of their precious companions.

During one of Mr. Bunting's failed attempts to capture Rudger, Amanda gets into a fatal car accident. All alone now, Rudger must protect himself from not only his pursuer but also another peril: if an imaginary friend is forgotten, they will completely disappear from existence.