(from: Wilkie Collins: An Illustrated Guide © Andrew Gasson 1998, used with permission)
In 1828, the firm of Wagner, Keller and Engelman has offices in London and Frankfurt. After the death of her husband, the progressive Mrs Wagner becomes senior partner, running the London office where she plans to employ women clerks. To prove that lunatics can be cured by kindness, she removes the simple-minded Jack Straw from Bedlam and takes him into her household.
In Frankfurt, Fritz Keller, son of one of the other partners, has fallen in love with Minna Fontaine, daughter of a sinister widow whose husband made a lifetime study of poisons. Keller's father disapproves of Madame Fontaine, who is constantly in debt, and refuses to allow Fritz and Minna to marry. Fritz is sent to London and a young Englishman, David Glenney, who tells the story, goes to Frankfurt in his place. Here he meets Minna and her mother and innocently introduces them to the third partner, Engelman. Madame Fontaine, determined to further her daughter's marriage, uses Engelman, who falls in love with her, to trick her way into Keller's house.
Madame Fontaine possesses a chest of poisons and their antidotes which her husband intended to be destroyed on his death. She doses Keller with a slow-acting poison and to win his goodwill revives him with the antidote. She becomes his nurse and then his housekeeper, having assured him, falsely, that she is no longer in debt. Keller withdraws his objections to the marriage of Fritz and Minna.
Mrs Wagner comes to Frankfurt on business accompanied by Jack Straw, who is much improved by her kindness and devoted to her. He is immediately recognized by Madame Fontaine as 'Hans Grimm', mentally damaged by being accidentally poisoned in her husband's laboratory years before.
Fritz returns to marry Minna. Madame Fontaine has a debt to pay which falls due the day after the wedding, but when the ceremony has to be postponed she steals the key to Mrs Wagner's desk from Jack Straw and embezzles the money she needs from the firm's funds. Mrs Wagner discovers the theft and, unable to pay back the money, Madame Fontaine poisons her with a fast-acting, undetectable poison. Jack Straw unsuccessfully tries to revive his benefactress with the same antidote that had saved Keller and her body is taken to the Deadhouse, where the devoted Jack refuses to leave her. In a lurid scene Madame Fontaine, who has secretly followed them, is accidentally poisoned by her own mixtures while Mrs Wagner recovers from a deathlike coma, ringing an alarm bell which alerts the watchman.
Madame Fontaine dies, leaving a self-incriminating diary. Fritz and Minna marry, while Jack remains in the care of Mrs Wagner.