When a nasty stomach bug sweeps through Portwenn, Martin takes to the radio to alert residents to the danger, with unforeseen consequences.
Louisa returns to Portwenn with a huge surprise and Martin is caught between two women.
Martin struggles with Louisa's surprise news, but Louisa has her own struggle when her return to school is made difficult by the increasingly strange behaviour of the headmaster.
Martin comes face-to-face with Danny, an old flame of Louisa's who has returned to the village to visit his ageing mother, but the two fail to get along. Plus, is there a mugger in the village?
Doc Martin has a patient catastrophe on his train journey back home from London and Aunt Joan is being sued by parents of a schoolboy who became ill after a visit to her farm.