At his Kansas City home, industrialist Curtis Langley Dodd is playing the piano, unaware that someone has hidden a crossbow inside of the instrument. Jim and Artie arrive at the front door and tell the butler, Griffin, that they're there to see Dodd. The butler refuses and Jim shoves his way past the butler and the two manservants. While Artie deals with them, Jim heads for the piano room and runs into Dodd's wife, Elaine. She tells Jim to leave but he warns that her husband is in danger and enters the room. Jim tells Dodd to stop and Griffin arrives to take him captive with a gun. Jim tries to disarm the butler and the crossbow goes off, killing Dodd.
Later, the agents return to the Wanderer to meet with their immediate superior, Colonel Crockett. The grieving widow comes to apologize and worries that she may have delayed Jim and caused her husband's death. Crockett assures her that she's not to blame and bids her farewell. Once she leaves, Crockett tells the agents that President Grant has sent his regards concerning Dodd's death, and that he's the fourth in a series of "alphabet killings." Each victim was a rich industrialist who died on a holiday, with Dodd dying on Easter. Crockett relays Grant's concern that if someone buys up the holdings of a few more industrialists once they die, that such a person could control the country's economy. As Crockett departs for a Senate committee meeting, Jim notices that the sympathy note that Elaine left has the same font and broken M serif that @@@@@
Artie confirms that the two cards came from the same shop, the Friendly Card Company in Kansas City. The agents break into the shop, unaware that someone is watching them through a hidden peephole. As they search the place, the agents find a wedding invitation from Miss Emma Valentine, announcing the marriage of Michele Le Master to a rich industrialist, Paul J. Lambert. The hidden observer fires a Cupid arrow at them from a sculpture of the cherub and three men emerge to attack the agents. One of them knocks out Artie and throws him onto a paper-cutting machine, and Jim breaks free of the other two so he can save his partner. Two of the thugs retreat through a secret panel and the agents capture the third one. However, someone outside the window shoots and kills the man before he can reveal anything.
The next day, Artie meets Jim at the train after learning that the owner of the card company is a man named E.N. Itnelav, and the dead thug has no identification. They wonder if Elaine set them up and check Lambert's wedding announcement. There's a newspaper article on the wedding and the fact that Emma Valentine, a Washington hostess who has been hired to manage Lambert's marriage. The agents do some digging and confirm that each of the murder victims had married a younger woman within the last six months. They figure that the killer used a fake alphabetic pattern to kill the first four men and distract the authorities from the fifth victim, Lambert, whose name doesn't start with an E.
Artie goes to see Lambert while Jim goes to see Itnelav at the card company. Itnelav claims that nothing happened there and when he came in, there were no signs of a fight. Emma Valentine arrives to pick up an order of cards and introduces herself to Jim. She invites Jim to the reception that evening as a novelty guest and Jim accepts. Meanwhile, Artie tries to convince Lambert to cancel the marriage, finally yelling at the man to get his attention. Lambert starts to agree but when his fiancée Michele arrives, the millionaire can't bring himself to disappoint her.
Emma is exercising in her bedroom, unaware that Jim has broken in the house. Michele comes to see Emma, who she is working with, and is upset at her role in having to deceive Lambert. Emma reminds Michele that she used to be a barmaid and soon she'll be married to the wealthiest man in America. A series of alarm bells go off as Jim unwittingly trips the alarms, and Emma is able to track his movement to the wedding chapel. She goes to meet him with her thugs and they quickly subdue Jim.
Jim wakes up in Emma's bedchamber, held in a chair by dummy women's arms capable of crushing him whenever Emma wishes. She explains that she has arranged the marriages and deaths of the four industrialists so that she can take control of the U.S. economy and then avenge the numerous slights of men against women. Each widow works for Emma, and now that they control their husbands' wealth, she controls them. She plans to have Congress appoint her Empress of the United States and then she'll assure equality for women everywhere.
Back at the Wanderer, Artie and Crockett try to work out their next move. When Artie sees Itnelav's card in a mirror and sees that it is "Valentine" backward, he realizes that Emma is responsible for the murders.
Emma shows off her Love Eternal Machine, a mechanical computer, and asks Jim about his description of a perfect woman. As he responds, trading her question for question, Emma cuts hearts in a piece of paper representing his answers and explains that she will arrange perfect marriages between men and women, and make sure that each person marries their perfect mate. She finishes cutting the paper and uses it as a punch card to enter Jim's preferences into her Love Eternal Machine, and it informs her that Jim's perfect woman is a paragon of three of the greatest women in history. Emma flirts briefly with Jim but when he rejects her, she slaps him and steps away. Michele comes in and Jim tries to undermine Emma, reminding the young wife-to-be that her boss is a murderer. Emma insists that Michele is loyal and then goes to see Lambert as he arrives for his fitting. Once Jim and Michele are alone, Michele tells him that she was born in France and had to steal once so she wouldn't starve, and that Emma has been blackmailing her ever since. Jim assures her that she's not a thief and to release him before she becomes an accomplice to murder. Before Michele can free him, one of Emma's thugs comes in to collect her.
Artie, disguised as a Jewish tailor, is measuring Lambert for his tuxedo as Emma looks on. The agent tries to stall the wedding, ripping the sleeves off of the industrialist's suit. When Elaine comes in and sees Artie, she whispers to Emma. As soon as Lambert leaves for the wedding, Elaine draws a gun and captures Artie, and Emma rips off the agent's false beard as her thugs surround him.
Emma has her men take the agents to the attic above the chapel and tie them to the stained glass roof. She then explains that the organ hammers are wired to key points on the glass. Elaine will play the organ and when she concludes with the proper, unique chord, the glass will shatter and the agents will fall to their deaths. The glass will kill Lambert as well, and the agents will be framed for his death. Michele will inherit Lambert's wealth and Emma will be able to declare herself Queen Emma. Elaine begins playing and Emma goes downstairs to attend the wedding.
In the chapel, Michele is escorted in to where Lambert is waiting for her. As the ceremony begins, Jim manages to severe a leg rope on a loose nail. He then uses his toe knife to cut his other leg free, and then swings up and cuts Artie's hands free with the blade. As the minister weds Lambert and Michele, the agents manage to reach their grappling hook guns and fire them into the ceiling.
Once the ceremony is concluded, Emma escorts Michele to the door and has Lambert pose for the first photo. Elaine plays the final chord and the stained glass shatters beneath the agents. They swing down on their lines and attack Emma's thugs. Emma tries to shoot Jim but Michele finally makes up her mind and knocks her arm up and away. The agents dispose of the thugs while Emma slips away. Emma gives the wedding ring back to Lambert and assures him that he won't have any trouble getting an annulment. Jim goes after and finds a message from Emma saying that she'll show that crime pays. He runs to the window and sees Emma get away in a coach.
Later, Jim and Artie return to the Wanderer after capturing Emma and find a heart-shaped box of chocolates on the table. It took them several hours to catch her and they admit that she could have had time to prepare a bomb and send it to them. Jim takes it to the window and throws the box of chocolates out, but nothing happens. He goes out to retrieve it and shows that Artie that it's now a demolished box of chocolates, a gift from Michele and an invitation to attend her wedding to Lambert.