In Murder Ahoy, Miss Marple is attending the annual board of trustees meeting of the organization she belongs to, the Trust for Merchant Seamen. During the meeting, a heated argument breaks out between the trustees, and one of them, Admiral Trevelyan, collapses and dies.
Miss Marple suspects foul play and discovers a white powder on the admiral's jacket that turns out to be a poisonous substance. She convinces the police to investigate, but they are skeptical of her theory. So, Miss Marple decides to take matters into her own hands and boards the ship where the admiral had recently been the captain.
She enlists the help of the crew and conducts her own investigation, leading her to uncover a web of secrets and lies among the suspects, who include the other trustees and members of the admiral's family. As she gets closer to the truth, Miss Marple becomes the target of the killer and must use her wit and resourcefulness to solve the case and bring the culprit to justice.
Murder Ahoy is a classic whodunit mystery with plenty of twists and turns, and Margaret Rutherford delivers a delightful performance as the eccentric and shrewd Miss Marple. The film is also notable for its picturesque setting on a ship, which adds to the ambiance of the story.