Danny Hayes is an awkward child who is afraid of the dark and the regular target of a local bully named Eric. Eric teases Danny about his interest in Lianna, a pretty student. At home, Danny's mother chastises him for sleeping with the lights on. A mysterious entity emerges from under his bed, proclaiming "I am the shadow man, and I will never harm the person under whose bed I live." The shadow man leaves the house.

The next day, Danny tries to tell his friend Peter about the shadow man, but Peter dismisses it. At school, Danny hears about a series of attacks on children; witnesses describe the attacker as a shadowy man. That night, the shadow man again emerges, reiterates his promise, and departs. Danny unsuccessfully tries to take a picture of him. The next day, Danny tries again to tell Peter, but Peter continues to dismiss it. Danny overhears that Lianna needs help in algebra, so he goes to her house to help. She is impressed that he would risk an attack by the shadow man just to help her.

Danny's "bravery" becomes the talk of the school, and he attracts a number of false friends. Peter, believing that Danny's story of the shadow man under his bed might be true, confronts Danny. Danny reacts dismissively toward Peter. Later, Danny challenges Eric to a fight, in an area where the shadow man has been most often sighted. At the appointed time, the shadow man arrives as expected, frightening Eric away. The shadow man begins to choke Danny, who, shocked, demands to know why the shadow man is reneging on his promise of never harming him. The shadow man acknowledges the promise to never harm the person under whose bed he lives, only to reveal "But I am a shadow man...from under someone else's bed!"

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Wikipedia