Mrs McGillicuddy sees a woman being strangled in a train travelling alongside her own, but no body is found and the railway records offer no obvious crime. Miss Marple calculates where the corpse may have been thrown and sends the capable Lucy Eyelesbarrow into Rutherford Hall to search.
The resulting investigation joins railway geography to a family waiting for an inheritance in a decaying country house. Post-war changes in transport, domestic service and property shape the case, while Miss Marple and Lucy expose the danger behind a household that treats an unknown murdered woman as less important than its own financial future.