This is the description of the episode from Wikipedia:
A children's magic show goes horribly wrong when one of the performers dies during a trick. Barnaby and Jones discover that the victim was poisoned with a rare toxin extracted from Ecuadorean poison frogs - so the hunt is on for a particularly ruthless and imaginative killer. The detectives soon learn that a feud is raging between local occult practitioner Ernest Balliol and famous writer Aloysius Wilmington. While some of the villagers - such as Ernest's daughter Isolde - believe that powerful magical forces are at play, others think the reasons for the bloodshed may have their roots in the distant past.While watching the show, I was puzzled by the various groups of people and their relationships. But that is one of the many good points of Midsomer Murders episodes. With 90 minutes per episode, there is time to have a realistically complex plot and tie it all togethr in the end.
Wilmington's nephew, Simon, is going through his uncle's library in search of rare books. Balliol's daughter, Isolde, seeks a secret text about dark magic in that same library. His son, Tristan, is a solicitor and cares for his mother, who has mental problems. The group that runs the magic show is linked to the pagan cult called the Temple of Thoth. Very confusing, but gradually it all falls into place.
Even though this was my second viewing of the episode, I was again surprised to find out who the murderer was. There were some wonderful scenes of the Englefield House in Berkshire and its grounds. This episode has a commentary with John Nettles and Jane Wymark (Joyce Barnaby), an unusual extra on a set of Midsomer Murders episodes.