From Paddington: A Miss Marple Mystery

Elspeth McGillicuddy was not a woman usually given to hallucinations. But when she witnesses what appears to be a woman being strangled on a train and no-one else sees it, no-one reports it and no corpse is found she turns to her old friend Jane Marple to help solve the puzzle. Marple asks her highly efficient and intelligent young acquaintance, Lucy Eyelesbarrow to infiltrate the Crackenthorpe family, who seem to be at the heart of the mystery, and help unmask a murderer.