The elegant Mrs. Lucas, known widely as Lucia, decides to leave Riseholme rather than see Daisy Quantock play the role of Elizabeth I in the village fete, a role that Lucia was forced to give up following her husband's death. Lucia finds a charming house in the village of Tilling-on-Sea, which is ruled by queen bee Miss Elizabeth Mapp.

Though happy that her friend Georgie is also taking up residence in Tilling, Lucia is dismayed that the vaunted piano at Mallards, the house she has rented from the formidable Miss Mapp, is unplayable. Worse yet, the Mallards' gardener is too busy tending Miss Mapp's vegetables to care for Lucia's flowers. Battle lines are drawn and the early advantage goes to Lucia. Miss Mapp plots her revenge.

Miss Mapp's scheme to reject the paintings Lucia and Georgie submitted for an exhibition backfires. But Lucia appears vulnerable when she schedules a social encounter with an Italian contessa. All want to know if Lucia can converse in Italian with the contessa or if the Italian phrases she and Georgie bandy about are pure affectation. Poised for victory, Miss Mapp is again outwitted by her rival.

Lucia stays on in Tilling at summer's end and purchases a house. As she moves back into Mallards, Miss Mapp seizes the chance to quiz Lucia's cook about her rival's lobster recipe but fails. Undaunted, Miss Mapp braves a storm to sneak into Lucia's new house to find her cookbook. Lucia appears as flood waters overtake the kitchen. The women upend a table to stay afloat and are washed out to sea.

With no news of Miss Mapp and Lucia all winter, their friends assume that both women have drowned. Major Benjy, Miss Mapp's heir, wastes little time moving into Mallards and issuing social invitations. Early one morning, Miss Mapp and Lucia reappear, and each of the intrepid castaways broadcasts her own particular account of the adventure. Before long, the battle between the two rivals resumes.

Tilling is atwitter with speculation about Georgie's mysterious illness. Lucia uncovers his secret and helps him escape his confined house. After Lucia invests in a gold mine, newlyweds Miss Mapp and Major Benjy also invest in the venture. When Lucia decides to sell her shares, Mapp and the Major take the opposite tack. The stakes rise when Mapp and Lucia vie for the same seat on the town council.