The film begins with a woman lying dead in the bathtub, having slit her wrists. It then moves on to show Karl (Aiden Gillen) at his wife's graveside, where his sister in law brings his daughter to see him. Karl asks her to keep his daughter for another day while he sorts out his apartment. He then leaves, and is shown driving down the road, where he receives a phonecall from the hospital, asking him if he would be able to come in and help with a patient. He declines, stating that he has something important to do for his daughter.Claudia (Amber Tamblyn) is shown at the hospital, worried about her grandmother's condition. A doctor appears and tells her that her grandmother has survived the surgery, but is old, so it is only a matter of time. Claudia leaves the hospital and gets on a bus.Tommy (Armie Hammer) is shown getting out of bed, where he wraps a bandage over his bruised knuckles. The woman sleeping beside him is bruised too. He gets on his motorbike and leaves.Karl pulls up at his apartment building and helps some elderly women leave the building. As he holds the door open, Claudia walks in and waits for the elevator to come. When it comes, the pair enter it, and the doors start to close, just as Tommy pushes the door open. Karl warns him about doing it, telling him that sometimes it can go wrong and cut people's arms off. As the lift is moving upwards, it gets stuck between floors, and all three begin to panic because they have somewhere to be. During this, Karl takes a photo of Tommy, but Tommy makes him delete it, causing an argument, where Karl punches Tommy, and Claudia has to break them up.A series of flashbacks then shows the reasons why everyone wants to get out early. Claudia is shown with her grandmother, who tells her that she isn't fun anymore, and that they should go and do something exciting. She realises that she is devoting too much time to work, and not enough to her social life, so she agrees to go to the beach. On the way, Claudia is stopped by a homeless man, but her grandmother carries on walking, and is involved in a car accident. After she wakes from her operation, she tells Claudia to go and fetch a photo of her husband so he can be near her when she passes. Claudia leaves to fetch the photo, and needs to get out in order to give it to her grandmother.Karl is shown taking a photo of a woman in a club. He begins talking to her and invites her to his house. He then drugs her, before carving marks into her chest. After this, he covers his body in salt and rapes her, before killing her. He leaves the mess in his apartment, and needs to get out in order to clean it up.Tommy is shown with his girlfriend Francesca, who is concerned he is just using her. Later that night, her father comes home drunk, and she tells him to leave in case he is caught, As Tommy is sneaking out, her father catches him, and she tries to calm him down, so he hits her.Tommy manages to knock him out, and tells Francesca that he wants to run away with her. She gives him her father's money and he goes home to retrieve some from his father's stash. He needs to get out in order to get the money.Back in the elevator, the three of them realise that due to the building being renovated, and people going on holiday, they are the only ones inside. Tommy pulls out a knife, and manages to wedge the doors open, but they are trapped between floors so they cannot escape. He decides to climb out, but as he is climbing to the next floor, the cable snaps, and he falls on to the elevator, which is now only held on by its emergency brakes. He climbs back in and they realise he has broken his leg.The three of them decide that someone should stay awake in case help comes to them. Claudia pulls a chocolate bar from her bag and Karl confiscates it from her, as well as taking her inhaler. This causes her to become agitated when he is smoking. During all of this, Francesca arrives and sees Tommy's bike outside. As she gets in, she realises no one is in, and is about to leave when she hears shouting. She decides to leave anyway, and returns home.While Claudia sleeps, Karl tells Tommy if they don't get out before his daughter gets there, he will kill Tommy and rape Claudia. Later on, he tells her that he has a plan to set the fire alarm off, if she climbs up and puts his lighter against the sensor, and if she fails, he will kill them both. He gives her inhaler back, and she climbs up, only to fall short of breath. She attempts to use her inhaler again but drops it down the shaft. She presses the lighter against the sensor, but cannot reach properly and falls on to the elevator, which has now become extremely vulnerable.She re-enters and Karl tells her he warned her what would happen, and he stabs Tommy to death. He goes to attack Claudia, but the elevator begins to fall towards an exit. He stabs her, but she climbs out into the corridor. He pulls himself out and starts wildly stabbing her, but she kicks him and he falls into the elevator, which drops to the bottom of the shaft and crashes. She then has an asthma attack.The next scene shows paramedics taking Claudia, and the girl from Tommy's apartment away, with Karl's daughter and sister in law in the police car. Claudia wakes up in the hospital, where the doctor who she spoke to earlier tells her that her grandmother didn't make it.
Output: Question: Who is sleeping when Karl reveals that he is a killer?, Answer: Claudia