When several members of the nobility are found floating in the Thames, Scotland Yard's Inspector Jack Ramsay fears they have been murdered by the notorious Rivers gang. He enlists the aid of Grace Fields, one of London's most knowledgeable botanists. A God-fearing woman, Grace defies the social mores of 1820s London and owns a shop selling herbal remedies and lotions. While she and Jack battle the Rivers, their initial respect and attraction firms into love.