Olivia and Theo are sent to spend the summer with their uncle, Professor Austin Hazelsmith, a terrifically inventive scientist. When the children arrive, they meet Lena Zelinsky, their uncle's unpleasant housekeeper and Vladimir Trokov, his creepy laboratory assistant. The pair tell Olivia and Theo that their forgetful uncle has left on a sailing trip and might be gone for months. Suspecting that things are amiss, the kids are determined to get to the bottom of the Professor's untimely disappearance. They join forces with Will, a neighbor boy, and enter a world of danger and intrigue when they discover the professor's own brilliant inventions have been turned against him by the strange "housekeeper" and "lab assistant." The devious Trokov and Zelinsky are holding him hostage with the goal of stealing the professor's extraordinary 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.