Due to their physician parents' professional obligations, Olivia and Theo are sent to spend the summer with their uncle, Professor Austin Hazelsmith, an eminent scientist and inventor. When the children arrive, they meet Lena Zelinsky, their uncle's creepy housekeeper, and Vladimir Trokov, his unscrupulous laboratory assistant. The two tell Olivia and Theo that their increasingly absent-minded uncle has left on a sailing trip and might be gone for months.

Sensing that things are amiss, Olivia and Theo join forces with Will, a neighbor boy, to uncover the truth about Professor Hazelsmith's whereabouts. They enter a world of danger and intrigue when they discover the professor and his former housekeeper are shrunken to doll-size and imprisoned in rodent cages. The devious Trokov and Zelinsky are holding them hostage with the goal of stealing the professor's scientific formulas and using them for evil purposes. The children, with bravery and brains, outwit Trokov and Mrs. Zelinsky in a spirited and exciting showdown.