Given a minimum of evidence, two Swedish boys investigate the laboratory and gymnasium of their high school to determine the identity of the prowler glimpsed in the hallway or the object of his wanderings. A button, a pair of galoshes and mysterious tapping sounds send Per and Jorgen on a precarious chase which Jorgen must finish alone because of Per's sudden illness. Taunted by the other boys for his suspicions, Jorgen proves his worth to the entire school population when he shows conclusively that one of the masters has been cribbing formulas from the talented chemistry professor by way of a concealed closet in the lab wall. The culprit's nervous habit of tapping his foot, the odor of shaving lotion and the fit of the boot add up to concrete evidence. Jorgen's search provides the only interest here since the reader never knows, until the very end, why an intruder might enter, what he might be after or who he might be. Poor motivation keeps suspense at a minimum.