Elspeth Inkpen is poisoned with aconite mixed into pesto sauce on a plate of tagliatelle. Aconite, a common wild and garden plant also known as Monkshood or Wolfsbane, is an extremely potent poison, known by the Greeks as the Queen of Poisons. The aconite used to poison her was picked from Elspeth's own garden.
Death by aconite poisoning is not pleasant, as DSI Barnaby mentions. The victim will experience numbness and tingling in the mouth and throat, both of which will feel parched. If the dose is large, it produces a severe burning sensation from throat to abdomen. The tingling feeling soon spreads to the hands and feet, and subsequently the whole body until the victim feels as though she is being flayed. This is followed by a loss of power in the legs and the dulling of sight and hearing, although the victim remains conscious until death comes. Muscular twitching may result in convulsions. Aconite will poison all the organs, but death is usually from cardiac arrythmia.