Miss Lucy Pym, a popular English psychologist, is guest lecturer at a physical training college. The year's term is nearly over, and Miss Pym — inquisitive and observant — detects a furtiveness in the behavior of one student during a final exam. She prevents the girl from cheating by destroying her crib notes. But Miss Pym's cover-up of one crime precipitates another — a fatal "accident" that only her psychological theories can prove was really murder.