School principal Mahesh Sidana, avid butterfly collector and founder of an elitist top-IQ society, is the first victim of a series of weird murders with an MO evoking the fate of caught insects. While Barnaby's Danish colleague-friend, detective Birgitte Poulsen, goes undercover in the IQ club, an interest of hers since her father's death, Barnaby and Jamie Winter follow other leads, such as Mahesh's widow's romantic interests, and other intrigues behind both hobbies. The IQ club president, suspiciously uncooperative, is abusive to his assistant, unsympathetic to the candidature of ambition-frustrated Carlton Scott's son Leo, and may be the recipient of ill-gotten gains.