In 1889 Cora Oakley died by inhaling a poisonous gas in her sleep, and her husband William was put on trial for her murder. Over a hundred years later, the only remaining members of the family are two elderly sisters who live in the ancestral home. Unable to maintain the mansion, the sisters decide to sell up and live off the proceeds. Then a young Polish man named Jan appears, claiming to be William Oakley's great-grandson and threatening to ruin the sisters' plans. When he is found dead, it seems that the shadow of murder has returned to haunt the Oakley family again, and Superintendent Markby must look back at the events of a century ago to find the killer...