Kate (Clémence Poésy) and Justin (Stephen Campbell Moore) are expecting their first child. They also have new downstairs neighbors, Jon (David Morrissey) and Theresa (Laura Birn), who are also expecting their first baby. Kate and Justin invite Jon and Theresa for dinner so they can get acquainted. During their dinner conversation, it's revealed that Kate and Justin originally didn't want children and were together 10 years before deciding to have a baby, and conceived rather quickly thereafter. Jon and Theresa tell them they struggled for seven years to have a child, and Theresa is visibly upset that Kate got pregnant as quickly as she did while she and Jon had tried for years. Eventually, Theresa, who has been sneaking wine all evening, says she doesn't feel well and gets up to leave. In her rush to get out the door, a tipsy Theresa trips over Kate's cat and falls down the stairs and subsequently loses the baby, whom she was going to name Peter.When Kate and Justin attempt to console the grieving couple, they end up in a fight with Theresa, who screams that Kate does not deserve "that thing" inside her. A few days later, Kate and Justin receive a note stating that their neighbors have left and will return when they can truly be happy for Kate and Justin's new addition.Kate gives birth and they name the baby Billy. Not long after Billy is born, Jon and Theresa return, apparently in good spirits. Theresa offers to babysit Billy so Kate can have time to herself. When Kate goes off to visit her dead brother's grave, Theresa leaves the house with Billy, but where she takes him isn't revealed. Periodically, Theresa and Jon are shown out and about with Billy. They're smiling and look like any happy couple with a new baby.Kate and Justin go to Jon and Theresa's apartment for dinner. Kate brings the baby monitor, through which she hears breathing. She rushes upstairs to find the bathtub overflowing. Over the next few days, Kate grows increasingly suspicious about the other couple's behavior. Kate leaves Billy with Theresa and then goes back to her apartment to spy on her. She sees Theresa take pictures of Billy and put him to her breast, apparently in an attempt to nurse him. Appalled, Kate breaks into Theresa's apartment and finds the camera with photos of Billy and photos of Jon and Theresa in public with Billy. She also discovers a fully furnished nursery complete with a framed photo of Jon and Theresa holding a baby, presumably Billy. Kate later drags Justin into the couple's apartment only to find that the photos have been wiped from the camera and the picture in the nursery has been changed.Kate, now on the verge of a complete breakdown, asks Justin to move to a new flat. They find a new place, and he works from home until moving day. This plan is cut short when he's called to work for an emergency. He hands Kate the milk, which she's often seen drinking, as he's leaving and tells her to call the moving van for tomorrow. When Justin reaches the office, he discovers there is no urgent meeting. He then receives an email from Kate stating she's "sorry". He rushes home to find Kate drowned in the bath, apparently having killed herself after tossing the baby into a nearby canal. Heartbroken, Justin moves out, as do Jon and Theresa.The flashback that follows reveals what really happened: Theresa and Jon drugged Kate via the milk from the bottle Justin handed to her on the way out the door; Jon sent Kate's "sorry" email to Justin from her laptop; and Theresa, dressed in Kate's clothing, threw a bundle resembling a swaddled baby into the canal. It's later intimated that the bundle was Kate and Justin's cat.The final scene shows Jon and Theresa in their new home in Germany, where they're settling with their baby, "Peter".