There's a new face in Sleepyside, and Trixie Belden is sure this stranger is up to no good. Too bad no one else believes her. But if anyone can get to the truth of this visitor's real motives, it's detective Trixie!

Trixie Belden hasn't talked to Diana Lynch in a long time--not since her family got incredibly rich...and snobby. But the week before Halloween, Trixie and her best friend, Honey, spot Diana looking a little down, and Trixie knows it is time to put old grievances in the past.

Except the past is exactly the problem for Diana. Her long-lost uncle Monty recently arrived from Arizona, and Diana is less than thrilled about the reunion. Monty won't stop pulling pranks, he is eager to buy lavish presents, and he constantly embarrasses Diana in front of all of her friends. Worst of all, he's insisting that Diana host a Halloween party.

Monty may have fooled the adults, but Trixie is convinced he's wearing a costume of his own. Can Trixie unmask Uncle Monty's tricks and prove he is an imposter--or will Di be the one who is sent packing?