When Lady Felicia's home comes under threat, Father Brown is forced to seek help from an unlikely ally. Lady Felicia is back from Northern Rhodesia to raise funds following a dreadful mining accident and is reluctantly selling a beloved necklace, otherwise she'll lose her ancestral home, Montague. A daring heist relieves her of the necklace and the prime suspects are the smooth-talking mystery Russian Nicholai Solovey and his henchman Yuri Galka.