Murder mystery. On the train to London, American mathematician Luke Williams meets elderly traveller Lavinia Fullerton. There have been a string of murders in her village, and she is on her way to Scotland Yard to reveal her suspicions about the perpetrator. But when the two arrive in the capital, Lavinia is mown down in a hit-and-run. Wondering if her ideas held any truth, Luke feels compelled to visit her village and investigate.