Vicki Benson is tenderizing meat for supper when her husband Adam comes home late from work. When she talks about his boss keeping him late, he snaps at her and then apologizes. As she explains that she accepted a party invitation for a friend, he loses his temper with her again. Vicki apologizes but Adam picks up the meat tenderizer and beats her to death.
Dean wakes up in the morning to hear Sam making a secret phone call. Dean feigns being asleep and Sam comes in to tell him that he's got information on three murders in Bedford where the loving husbands brutally killed their wives. The brothers visit Adam in jail and he explains that he loved his wife and has no idea why he killed Vicki. He says he didn't feel possessed and they were happy. Dean reveals that Adam has $9,000 in credit card charges to a strip club, the Honey Wagon. He admits that he met a stripper there, Jasmine, who he fell in love with during a bachelor party. She told Adam that they could be together forever if he killed his wife. Adam admits he has no idea where Jasmine lives or what his real name is. He also says that he deserves to die and if he doesn't get the death sentence, he'll do it himself.
Pretending to be an FBI agent, Sam drops in to see Dr. Cara Roberts, who works with the sheriff's department. He asks her about the workups on the three killers and their victims, and reveals that the men had high levels of oxytocin hormones. Oxytocin causes love-type feelings. Dean comes in but Cara seems more interested in Sam. She admits that she has no explanation for the high levels of oxytocin.
A disgruntled Dean complains briefly to Sam about Cara, and then explains that the other two murders hid away funds to pay for their strippers, and frequented a club called the Honey Wagon. However, each man had a different stripper, perfect. They go to the club but the owner doesn't have any paperwork on any of the three strippers. Sam has talked to Bobby, who thinks that sirens are involved. Sirens are creatures that lure men to their deaths, and can read minds to give their victims a perfect mate. They figure it's a single creature, but they have no way to determine who in the club it might be.
In a booth, Lenny Bristol meets with an attractive stripper, Belle. They go to Lenny's place and he makes sure his mother is sleeping peacefully. After sex, Belle tells Lenny to beat his mother to death so they can be together always. He goes into the next room to do so and Belle leaves.
The next day, Dean is alone in the hotel room doing research and notices that Sam forgot his phone. Dean dials the last number and hears Ruby's voice. He hangs up and puts the phone down as Sam returns to tell Dean about Lenny. Bobby calls and tells them that he's determined a siren can be killed with a bronze dagger covered in the blood of someone affected by her song. He figures the "spell" is some kind of venom and it will kill her if she gets a dose. However, none of the guys in jail are under the siren's spell any more. Bobby warns that sirens can turn men to her advantage.
Sam and Dean visit Cara again and Sam asks her for a blood sample. They're interrupted when a real FBI agent, Nick Monroe, arrives, claims he's been assigned to the case, and asks for their IDs. They tell him to call their supervisor, Mike Kayser, and provide a phone number. Fortunately, they have things set up with Bobby to take any such calls and pass himself off as a FBI director. Monroe brings them up to speed and explains he's discovered all of the murderers had strippers at the club. Sam tells Dean to take Monroe with him to keep him busy while Sam gets the blood sample from Cara. Dean reluctantly agrees and fortunately, Monroe is impressed by the Impala. Meanwhile, Sam talks to Cara who reveals that the blood sample has disappeared.
At the club, Dean and Monroe bond over drinks, classic rock, and strippers. Monroe reveals he found a purple flower at every crime scene and Dean remembers where he's seen such flowers before: at Cara's office.
Sam and Cara go over the security tapes but can't find any indication of who stole the blood. Sam explains his theory that the men were drugged and Cara talks about how sometimes you can hate the person you love. She pours out whiskey for both of them and talks about her failed relationship with her ex-husband. Sam's phone rings but he ignores it, and Cara admits she's been thinking about him all day. She hits on him and they end up making love in her office.
Sam returns to his hotel room and calls Dean, who explains that Monroe found hyacinths at the crime scene that originated in the Mediterranean. Dean has checked on Cara and she's only been in town two months, and her ex-husband died of a heart attack. Sam doesn't believe it and Dean figures he had sex with Cara. Sam insists he feels fine and asks to meet, but Dean insists on handling it on his own. Sam angrily throws the phone away. Dean leaves a message for Bobby and then calls Monroe asking for his help.
Monroe watches the club and sees Cara arrive. Dean arrives and says they'll wait to see who she comes out with. As they pass a flask back and forth, Monroe is skeptical of Dean's "poison" theory, and then notes she might be using her saliva. "Monroe" then tells Dean that he can't trust Sam and needs to get him out of the way so he and Monroe can be brothers forever.
Sam returns to the hotel room and finds Nick waiting for him. Dean emerges from behind the door and grabs his brother, holding a knife to his neck. "Nick" assures him that he has total control of Dean because he gave him the brother he always wanted. The siren admits he's bored and wants to fall in love again, then spits its venom into Sam's mouth. It then tells them to fight it out and the winner can be with it forever. Dean says that the Sam he knew is gone and that he knows Sam called Ruby. Sam finally tells him that they're hunting down Lilith because Dean is too weak and Sam's the better hunter. Dean attacks him and the two of them battle. They go through the door out into the hallway and Dean grabs a fire ax. Nick urges Dean on Dean starts to swing… but Bobby arrives and stabs him in the shoulder. Nick tries to run but Bobby throws the knife into the sire's back, killing it and freeing the brothers from its spell.
The next morning, Sam and Dean thank Bobby, who notes he easily figured out Monroe was a fake after one call. He wonders if they're going to be okay and they say they will be. He points out that there's no shame in a siren getting to them. After Bobby leaves, Sam says he's not interested in Cara and there's no point. Sam apologizes for what he said and Dean says the same, and they both claim that they're good.