S11E5 : The Magician's Nephew

Jean Wildacre dies suspiciously during a performance as assistant to local magician Aloysius Wilmington, and Barnaby & Jones are called in. They have to investigate the relationships of Midsomer's Magic Circle, a group of old friends whose activities spun off a new cult, and led to long-running animosities. Soon they are looking for colorful South American poison-dart frogs, as the death count rises