|In Cicero, Indiana, Dana Keel is waiting for Katie, her daughter, to arrive home. She has been staying with her father, Richard, but suddenly demanded to go home, telling her mother that she hated being with her father and that she is scared of the monsters. Her father arrives back home and is in his garage/workshop finishing up a rocking horse he is making. Suddenly, the electric power saw starts up by itself. He switches it off, but it happens again. Taking a closer look, something pushes him and he lands on the saw, the blade ripping through his body.
Elsewhere in Indiana, Sam is in a cafe on the phone to Bobby. They are still trying to come up with a way of saving Dean, without him finding out. Dean tells Sam about an article he read in a local paper about the guy falling on his saw. Sam isn't convinced that this is a case, but Dean reveals that there is something more in Cicero than just the dead guy - Lisa Braeden. Nearly nine years earlier, while Sam and John had been off on a job, Dean had hooked up with Lisa and claims it was one of the best times of his life. Sam agrees to head out there and checks into a motel while Dean drives out to the Morning Hill community where Lisa now lives.
Knocking on her door, Dean surprises Lisa, who is having a birthday party for her son, Ben. The boy, who is eight years old, bears a striking resemblance to Dean - he loves AC/DC, cars and chicks!
Lisa is indoors talking to Dana. Dana believes there is something wrong with Katie, more than just the loss of her dad. She has the idea that Katie isn't herself, but Lisa doesn't understand what she means.
Dean talks to Lisa about Ben, but she insists that Ben is not Dean's son. Dana takes Katie home, while Lisa tells Dean about the other deaths in the community.
Sam is in a cafe, doing some research, when the unknown hunter girl suddenly comes in and sits down. She says she is following Sam because she's interested in him, because of the whole "demon anti-Christ" thing. She mentions Sam and Dean's mom, saying something about her friends. She gives Sam her number and tells him to do some looking into Mary's friends. As she leaves, Dean phones and tells Sam about the other deaths.
Dana has fallen asleep on the couch. When she wakes up, Katie is sitting on the edge of the couch and demands that her mother plays with her. She hugs her mom, telling her she loves her. Dana sees their reflection in the mirror and is horrified to see her daughter looking monster-like. Meanwhile, Sam is investigating one of the other deaths, where a man fell out of a second storey window. He notices the daughter of the man inside the house, staring at him in a very creepy way. He also sees what look like bloody marks on the windowsills and finds out that the daughter was the only one there at the time of the accident. He also notices a strange mark on the back of the mothers neck.
Dana is making toast for Katie and can feel a mark on her neck. She locks herself in the bathroom to have a look, but Katie starts hammering on the door, trying to force her way in. The doorbell rings and Katie suddenly stops her noise. The Realtor of the community is at the door wanting to know if Dana will be selling the house. Dana tells her it is a bad time and slams the door. Katie demands ice-cream.
Dean sees Ben sitting alone on a bench and finds out that bullies have taken his game console. Dean offers to go get the game back, but Ben says no - only ******* send a grownup to sort out their problems. At Dean's encouragement, Ben goes up to the bully and asks for his game back. When the bully refuses, Ben kicks him and takes the game back. Dean is impressed, but Lisa has seen it too, and is not quite so happy. Lisa pulls Ben away, but the boy runs back to Dean and hugs him, thanking him for the help. As they walk away, Dean notices the other children reacting strangely.
Dana gets Katie into the car so they can go for ice-cream, but when she looks in the rear view mirror, she sees only a hideous monster in the place of her little girl. She drives to a large lake and gets out of the car. Leaving the monster that is posing as Katie in the car, she takes off the hand break and lets the car roll into the water. In tears, she gets back home, only to find the same creature, still looking like Katie, sitting in the kitchen, dripping wet, asking if it is ice-cream time yet.
Sam is researching Changelings, who can perfectly mimic children. They changelings have been grabbing the kids and posing as them in order to suck the synovial fluid from the mothers until the mother finally wastes away. Fire is the only way to destroy them. The real kids are probably still alive. Dean insists on going to Lisa's house and checking on Ben. But it looks like he is too late...
The same red marks are on Lisa's windowsills that Sam saw earlier, but Dean doesn't think it's blood. It's actually red dirt from one of the houses that is still under construction. They enter the house and Dean finds the kids in the basement, locked up in small cages. Sam finds the mother-changeling who has taken the persona of the Realtor. Sam uses the flamethrower against her and buys them some time, while Dean releases Ben and the other children from the cages. He breaks a window and starts getting the kids out, but the mother-changeling reappears in front of them.
Dana and Lisa are both now aware that these are not their children and are trying to keep as far away from them as possible.
Dean and Sam fight off the mother-changeling, while Ben helps the rest of the kids to escape. This time, the flamethrower works fully and the mother-changeling is destroyed. The changeling children burst into flames and vanish too. Ben is reunited with Lisa and Dean tells her the truth about what happened. Dean asks again if there is any chance that Ben is his, but Lisa reassures him, saying that the father was a guy she met in a biker bar. Dean is actually a little regretful that Ben isn't his son - he even says he would have been proud to be his dad. Lisa offers to let him stay a while and get to know Ben, but Dean says he has too much work to do and leaves.
Meanwhile, at the motel, Sam is looking into what happened to Mary's friends. On the phone, he receives details about several people that Mary knew. They're all dead. Every one of them, including her uncle and her doctor.
Sam calls the unknown hunter back and talks to her. Someone was systematically killing all of Mary's friends. He wants to know why, and he wants to know how the hunter knows information that he didn't. Suddenly, her eyes turn black - she's a demon! Dean is about to kill her, but she insists that she is there to help him. She cant answer all of his questions, but she is willing to help him figure it out, for reasons of her own. In exchange for working together, she offers to help save Dean.