Witchcraft is the talk of Midsomer when tensions flare between former friends Aloysius Wilmington and Ernest Balliol. Ernest, who runs a store selling magical artefacts, is a firm believer in a cult called the Temple of Thoth, but Aloysius has upset him by writing a book claming the dark mythology was his own drunken creation.


Desperate to prove the cult is real, Ernest wants to locate a medieval text about the Temple in Aloysius's library. Aloysius's nephew Simon is cataloguing the collection, whilst also stealing some valuable editions to pay off his debts to antiquarian bookseller Hugo Cartwright.


Isolde Balliol, Ernest's daughter, tries to charm Simon into helping them, while his son Tristan shoulders the burden of caring for their mother, who is losing her mind.


Tragedy strikes during Aloysius's magic show, when his assistant Jean Wildacre drops dead as they perform a trick inside the 'Chamber of Death'. A broken ritual knife is found in the casket.


As Barnaby and Jones investigate, the members of the Magic Circle all seem to be hiding something. Hugo tells Simon that Jean knew he was selling Aloysius's books.


A post mortem reveals that Jean was poisoned by a deadly toxin secreted by a tropical frog. Aloysius, who lived with tribes in South America, realises he is prime suspect - but also wonders if he was the intended target.


Simon finds an ancient manuscript which could be what Isolde and Ernest are seeking, as Hugo shows Aloysius a broken ritual arrow he received in the post. The next morning, Hugo is found dead with cuts on his hands - another victim of frog poisoning.


As Barnaby goes through old letters kept by Jean and Hugo, he makes a shocking discovery about Aloysius. But will he manage to find the killer before another member of the Magic Circle dies?