When shy, anxious Milo rings the doorbell of the forbidden Shock City castle on a dare, he gets more than he bargained for when peppy young monster Sunny answers the door and labels herself Milo's new BFF. But when Milo returns to tell Sunny he—and the rest of the town's residents—are moving out of the crumbling Shock City, his monster pal refuses to accept it. She soon convinces Milo to join her on a romp around the city to reverse the damages done, and persuade him and everyone else, to stay. But when Sunny learns that her grandfather, the great Darkness Von Shock, was not the revered benefactor she believed him to be, and is actually the cause of everything bad happening in Shock City, Milo confronts his biggest fears to help Sunny save the town and reverse her family's reputation.