The Shell Seekers is a novel that revolves around the final months of the central character, Penelope Keeling. Although the story focuses on Penelope, there are various glimpses into other characters that are important to her. As the mother of three children named Nancy, Noel and Olivia, each of the children's lives in the present are explored briefly in order to better explain their relationship with Penelope.

Penelope has just suffered a slight heart attack at the beginning of the novel and she is happy to return home (after voluntarily checking herself out of the hospital.) She then decides that it is time for her to return to Porthkerris, where she lived throughout World War II and met the love of her life, Richard. She asks each of her three children to come with her to make the trip, but all three decline, stating other obligations in their life. In the end, she decides to take Antonia, the daughter of Cosmo, the love of Olivia's life. Cosmo has recently passed away and Penelope takes her into her home in Podmore Thatch as a welcome visitor and an easy way to silence her daughter Nancy's constant admonishings that she needs a full-time caretaker. In addition, Penelope and Antonia take Danus, the gardener whom Penelope hires after suffering her small heart attack.

Central throughout the story are the paintings made by Penelope's father, Lawrence Stern. Lawrence was a famous painter, although over the years the popularity of his paintings diminished. Recently, however, there has been an increased admiration and appreciation for his works. Due to their rarity, the price for these pieces are astronomical. Since Penelope owns two incomplete panels in her home and one important work—The Shell Seekers—her children Nancy and Noel urge her to sell the works while their popularity has reached its peak. Penelope balks at the idea since The Shell Seekers is such a sentimentally important work for her; no amount of money can compensate for simply having it in her home.

After having another argument with Nancy and Noel about selling the pieces, Penelope finally goes to an art dealer unbeknownst to her children. She sells the panels for one hundred thousand pounds and uses the money to take herself, Antonia and Danus to Porthkerris, staying at one of the finest hotels in the region, The Sands. The only child that understands her actions is Olivia, but the other two are outraged. They become even more angry when they learn that Penelope donates The Shell Seekers to the art museum in Porthkerris that her father founded rather than sell it for an outrageous financial sum.

Shortly after Penelope returns from Porthkerris, she settles into her house again with Antonia. However, after a restless night of sleep, she goes out to her garden and suddenly suffers another heart attack. This time she does not recover, but in her final moments, she sees Richard coming for her, filling her with happiness. The children gather together for the funeral and hear her final thoughts in her will. Danus is granted a large sum of money from the sale of sketches by Lawrence Stern that Noel had wanted and Antonia is given a set of lovely pearl earrings that Nancy had wanted and yet again, these two adult children whine over what their mother did not do for them. In the end, however, Olivia silences all of them and allows Antonia and Danus—by now a symbol of Penelope and Richard—to start their new lives together with the money that Penelope had bequethed to them. It is a realistic story about families and the intense relationship between a mother and her children, as well as how adult children face their mother's imminent death.