The headmaster of a local school is murdered, his body found pinned to the wall in the manner of his own butterfly collection. The dead man was a mysterious international club for geniuses, which are currently testing the school's pupils for possible new recruits. Barnaby and Winter investigate the secretive organisation, which has proven controversial in the local area, but also uncover the possibility of a connection between the killing and the impact of pesticides on a rare species of butterfly.