Jerry, 11, glimpses two child-size apparitions at the local museum and is instantly intrigued. When they ""summon"" him to be their playmate, he falls a little in love with the older ghost, Mattie, and is charmed by her smaller brother, Edward. Lonely Jerry at first intends to become famous for discovering the ghosts; then his crush on Mattie makes him realize that she needs someone to take care of her and Edward; finally, he understands that they have other worlds to which they must tend. In the meantime, he finds himself involved with their sinister uncle, who, in his quest for immortality, is responsible for their deaths and needs Jerry for further experiments. This slight ghost story is redeemed by the well-developed characters of all three children and by Jerry's relationship with Mattie and Edward. His motives run a tremendous range, all naively appropriate for a boy his age, and give the story scope. The evil uncle escapes--to a sequel?--but there's a satisfying sweet resolution.