Green-fingered amateur detectives Laura and Rosemary are called in to tackle a stubborn species of weed threatening an entire year's vintage at the Crickle Valley vineyard. However, they find themselves in the right place at the right time when the body of a wine critic is discovered on the premises.

The duo help reorganise the seed museum owned by the descendants of famous botanist Edwin Pargeter - but it is not long before a man is found dead in the family's garden. In an effort to trace his killer, Rosemary involves colleagues from the Royal Horticultural Society at Kew Gardens, and soon the race is on to identify a batch of mysterious seeds before it falls into the wrong hands.