Stabler and Benson are investigating the double murder of a couple in an apartment building belonging to an eccentric former Rockette dancer, Birdie Sulloway. She invites them into her home and asks her nephew, Chet, to fix them a martini. The detectives decline, they are more interested in solving their case. They discover that Birdie's husband, Marv, owned the Ballerina Room inside the building and where one of the victims worked as a dancer.
Marv frequented the club often and obsessed with Tisa. Birdie and Chet volunteer to come in and talk to Benson about Marv's carelessness when it came to their investigation. Birdie offers to pay for Tisa's funeral as an apology. She also disposes of a chewed-up pen of Marv's that Benson sneaks into her bag as DNA evidence. Benson also finds out that Chet isn't really Birdie's nephew, but was taken by in by Birdie when he was just six, after his mom died.
Marv's DNA from the chewed-up pen matches the DNA from Tisa's rape just before her murder. The detectives head over to Birdie's to make an arrest. Before they can reach the door, Marv's body falls from the penthouse onto a parked car. He's dead and the detectives rush upstairs. Chet and Birdie both claim that Marv was in his study alone before he jumped, but fingerprint evidence points out that Marv was pushed out the window, forcefully.
Chet and Birdie both come in to the SVU for questioning. Cragen tells the detectives that Marv had large amounts of anti-depressants in his system at the time of his death, so if they are wrong about their murder theory, they are going to have some tough questions to answer. Birdie admits that Marv was on all types of medication and it always concerned her. She snaps at Stabler when he tries to get more answers as to why Marv was on so many pills.
Chet gets wildly defensive of Birdie, claiming to be the only man who has always been there for her. Benson fills Chet in on the details: Marv was being investigated for double homicide and rape of one of his dancers. Chet is completely shocked. Benson wants to know the intimate details of Chet and Birdie's relationship. Birdie tells Stabler that she and Chet were also lovers, starting when Chet was 18.
Benson accuses Chet of spiking Marv's drink and pushing him out the window so he could have Birdie all to himself. Chet freaks out and wants to find Birdie, but Benson assures him that she is busy telling her side of the story to Stabler. Chet knows Birdie would never turn on him. Benson shows him Birdie's interrogation room and says that she's telling Stabler everything. Chet can't believe what he's seeing as he watches Stabler and Birdie having a conversation from behind a double-sided mirror. He accuses Birdie of pushing Marv out the window.
Huang wants to have a psych evaluation of Birdie and immediately becomes suspicious of her motives. She begins twitching and complaining of aches and pains. Huang reports back to Cabot that Birdie shows signs of a serious neurological disorder and that might put a blow in her case.
Birdie undergoes a CAT scan to figure out what's wrong with her. The CAT scan confirms that Birdie is suffering from brain cancer that has gone years without being detected, due to her disdain for doctors. Huang explains that the cancer would explain Birdie's mood swings and fits of rage.
Huang delivers the news that Birdie has brain cancer and even the most radical chemotherapy can't help the fact she only has months to live. Cabot realizes she can't try her case against Birdie because she has already been given a death sentence. Huang breaks the news to Birdie, who is saddened.
Birdie starts talking about death and how she'd become fascinated with it since watching her third husband die in her arms. Benson listens in and tells Cabot that Birdie's original story was that her third husband was swept off to sea on their yacht. Benson tells Cabot she believes Marv wasn't Birdie's first victim.
Stabler and Benson do their research and discover that all four of Birdie's husbands died under mysterious circumstances. Each husband had a rap sheet, making Fin suspect she was more of a vigilante murderer, than a black widow. With her lawyer present, Birdie confesses to ridding the world of five horrible men. Birdie begins to hint that Chet was the real person who carried out Birdie's wishes by helping her commit murder.
Chet pays Birdie a visit in the hospital room and is shocked to see her condition. She tells Chet that she's appreciative of everything he's ever done for her and Chet tells her he would do it all, again. Including pushing Marv out the window.
Birdie flirts with Chet and then tells him he's about to be taking a little trip. He looks confused. The detectives walk in and arrest Chet for murder. He's confused on why Birdie turned, but she tells him that in the 30 years of knowing her, he should have learned to never cross her. She tells Stabler and Benson that everybody does what they have to in this world.