Retired classics teacher Madeline Villiers goes to a remote jungle in Borneo with her lover, botanist Jimmy Fong, to smuggle out a yellow Roth orchid, one of the rarest in the world.

Back in Midsomer Malham, it's the annual flower and orchid show hosted by the Rt Hon Munro Hilliard and judged by charismatic Professor Margaret Winstanley (Harriet Walter) and her research student Jonathan Makepeace. In the orchid marquee, competition is intense.

Compulsive collector Henry Plummer is angry at coming third but is soon cheered by a priceless new arrival - the Paphiopedilum Rothschildianum Flavum taken from Borneo. But the next morning, Madeline Villiers is found dead.

As Barnaby and Scott investigate, it appears that Miss Villiers was leaving to get married in Switzerland. In her safe are notebooks, written in Latin. Local monk Brother Robert (Geoffrey Chater) translates - Miss Villiers was selling rare smuggled orchids and writing a lurid account of her sex life.

Henry Plummer is found hanging in his garden shed, his orchid collection destroyed, while tests reveal Miss Villiers was poisoned by hemlock. The detectives go to find Munro Hilliard, who is guarding a rare monkey orchid on the downs, but when he waves a gun at them, Scott accidentally steps on the plant.

Handyman Harry Rose was jailed for killing another man but denies anything to do with the new deaths. But he leads Barnaby and Scott to Miss Villiers' incinerator containing a charred medicine bottle and a bouquet of white flowers.

While the detectives hunt for Jimmy Fong, Hilliard is speared through the chest with a garden fork. He had just purchased the yellow Roth - but it has now disappeared again. Brother Robert's translations reveal the full extent of Miss Villiers love affairs - but who is prepared to keep killing just to get their hands on a rare and delicate plant?