A body of a woman is found in the river by a swimmer. Carella is assigned to the case and is told by the medical examiner that the girl was dead before she hit the water due to arsenic poisoning. The detective is also informed that the girl was about 30, had an appendectomy scar, and had a heart-shaped tattoo between her thumb and right forefinger that said "MAC."
Carella and Meyer check the missing persons files and find one that matches the victim exactly except for the tattoo. The name of the missing girl is Mary Louise Proscheck, a girl who recently moved to Isola from Scranton, PA. Before leaving Scranton, Mary left her parents a goodbye note saying she was leaving to find a husband. Mary's father arrives at the precinct a few days later and identifies his daughter from a charm bracelet that was taken from the body.
Later that evening at his home, Carella is in a somber mood due to the events of the day. His deaf mute wife, Teddy, draws him out of his bad mood and shows him her copy of Pen Pal magazine that she was reading for laughs. The camera then focuses on a personal ad placed by a man looking for "an understanding woman of good character."
On the following day, a couple are seen takiing a romantic carriage ride in the city. The woman, whose name is Priscilla Ames, asks the man how a good-looking guy like himself could be attracted to someone as homely as her. The man, whose name is Curt Donaldson, tells her that he is interested in an intelligent woman like her not some bikini clad poster girl and that's why he placed a personal ad in Pen Pal magazine.
Later, Carella and Teddy are headed for lunch when they stop at a tattoo parlor to see if the owner knows anything about the tattoo they found on murder victim Mary Louise Proschek. The owner knows nothing about the tattoo but asks Teddy if she'd like to get one. She is interested but her husband demurs. In the meantime, Detective Bert Kling is checking out another tattoo parlor and hits paydirt. The owner remembers the the tattoo and the victim and says she had a boyfriend named Curt. That's what the MAC stood for--Mary and Curt.
During lunch, Carella has to fight off the attentions of a drunk who starts hitting on his wife and then takes a phone call from Kling about the description of the suspect. Back home, Teddy is still thinking about getting a tattoo. She returns to the tattoo shop she and her husband visited earlier and asks for a butterfly tattoo on her shoulder. The shop owner, who calls himself Charlie Chan, agrees to put the tattoo on her immediately but they are interrupted by Curt who enters the store and asks about getting some tattooing done. Charlie Chan tells him to wait thirty minutes and then he will be available.
Cut to Curt and his girlfriend, Priscilla, at a restaurant where Curt slips a dose of arsenic into Priscilla's drink. Cut back to the tattoo parlor where Curt and Priscilla arrive just as Charlie Chan is finishing Teddy Carella's tattoo. Chan goes out to greet them while Teddy remains in the back admiring her new tattoo. Curt asks for a tattoo just like the one Carella inquired about earlier. This immediately rings a bell with Chan and he rushes to the back and informs Teddy that the man her husband is looking for is in the shop. Teddy gives Chan the number of the precinct but Curt and Priscilla depart the shop due to the fact that Priscilla is becoming ill from the arsenic. Teddy decides to follow them and hails a cab.
Teddy and the cab driver follow Curt and Priscilla to the waterfront docks. Priscilla is now very ill from the arsenic. Teddy instructs the cab driver to call the 87th Precinct while she continues to trail the couple. Unfortunately, she is spotted by Curt and grabbed by him.
Meanwhile, the cab driver has notified the precinct where he is put through to Kling. Kling then contacts Steve Carella at his home and they agree to meet at Pier 7.
Cut back to the pier where lover boy Curt is attracted to Teddy. He says that maybe she's the one who could finally make him happy. Teddy manages to escape and runs away taking shelter behind some crates. The cops show up and find Priscilla who is still alive. An ambulance is sent for but where is Teddy?
Lethal Lothario Curt has finally tracked Teddy down but hears someone approaching. He then pulls a knife but Teddy knocks over some crates in order to warn the approaching person. It is her husband. Seeing this, Curt decides to get the hell out of Dodge but his escape route is cut off by Meyer. Carella closes in on Curt from the other direction. They fight and end up taking a fall off the pier into the water. Meyer throws Carella down a bailing hook in order to remove the garbage.