London, 1923. Returning from Paris, botanical researcher Saffron Everleigh finds that her former love interest Alexander Ashton's brother, Adrian, is being investigated for murder. A Russian scientist working for the English government has been poisoned, and expired in Adrian's train compartment. Alexander asks Saffron to put in a good word for Adrian with Inspector Green. Despite her unresolved feelings for Alexander, Saffron begins to unravel mysteries surrounding the dead scientist.