On assignment to prove leprechauns aren't real for his cable television show, I Don't Believe It!, Cassie's dad takes the Strange family to the Golden Shamrock Resort, which is supposed to be riddled with leprechaun trickery. Hoping to be the envy of her friends by staying at a luxurious spa, Cassie is sorely disappointed to discover her "posh" accommodations include a barn and a catfish-infested pool.

Determined to have fun, she and her brother Lucas make friends with another guest named Hank. Before long, Cassie discovers that Hank is actually a prank-loving magical being. He draws her into a game that traps her and Lucas in his secret underground world. Hank wants them to be his friends forever and the longer they stay in the underground paradise, the quicker their own world fades from memory. Will they be trapped forever? Or can Cassie and Lucas help Hank make friends in his own world so they can return to theirs?