Mum & Dad

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The film begins at Heathrow Airport, where a Polish immigrant named Lena (Olga Fedori) is working a shift as a cleaner. While there, her colleague Birdie (Ainsley Howard) helps her to clean the toilets. The two begin to talk and Lena reveals that she lives alone and doesn't keep in touch with her family. Birdie introduces Lena to her adopted brother Elbie (Toby Alexander), who also works at the airport and is a mute. Lena tells Birdie that she doesn't get on with her family. While talking Lena notices scars on Birdie's arm, Birdie explains that she used to have behavioral problems; but that she's better now. At the end of the shift Lena misses her bus so Birdie tells her that her dad will drive Lena home if they walk to Birdie's house.
Upon arriving at the house Lena is knocked out from behind and injected with a syringe. She wakes up tied to a chair to the sound of screaming coming from another room. Moments later Mum (Dido Miles) and Dad (Perry Benson) enter the room and introduce themselves. Mum tells Lena that if she does what she is told everything will be ok. Lena is unable to respond because of the injection she received earlier and Mum proceeds to inject her with a sedative. When she wakes up again Mum begins carving marks into her back. Lena is unable to speak, and Mum tells her to not try to talk. (Because she has had a little jab.) She gives Lena another injection, and Lena falls asleep. She wakes up later, tied to a frame. Mum tells her that she wanted another girl to come and live there. Mum then pierces Lena's skin with some metal and carves some marks into her back. Following this incident Lena is taken to see Dad who tells her that in his house she will follow his rules
At breakfast, the family are watching pornography. Lena makes an escape attempt but Dad grabs her and Elbie drags her back to the table. Birdie takes Lena through her chores. Back in her room, Lena attempts to escape, but hears Dad watching her through the keyhole. Over the following days Lena is repeatedly humiliated and tortured by Mum and Dad often for no particular reason while Birdie and Elbie attempt to get her in more trouble. Lena finds and hides a mobile phone but is quickly caught and put on heavier sedation as a consequence. A few days later Lena catches the attention of a man outside but is seen by Dad who drags her into his torture room, puts her in a suitcase and orders Elbie to hit it with a hammer. Meanwhile, the man enters the house but Mum is waiting and suffocates him with bubble wrap. The family then dismember the man and make Lena kiss his severed head. They then proceed to cook the man's appendages as sausages and eat them. As further punishment Lena spends that night tied to the kitchen radiator.
The following night Lena manages to break free from the restraints of her bedroom while the family is sleeping. She makes her way to the torture room and finds a staircase up to an attic, where she finds a mentally ill girl in a bed. The girl begins to struggle but Lena consoles her. In another room she finds a man who is being restrained, he too begins to struggle, almost getting her caught but calms him down and finds a weapon.
The next morning, Birdie enters Lena's room and tells her it's Christmas. She goes downstairs to find the man crucified on the wall, and the mentally ill girl in a chair. Dad tells Lena that he knows she broke free the previous night and saw the girl. He explains that the girl is his daughter and that she was a "spastic" because the umbilical cord got wrapped around her neck during birth and he had to bite it off with his teeth, implying that the birth didn't occur under medical supervision. The family proceeds to open a series of inappropriate Christmas presents. However, Mum neglected to buy a present for Dad so she tells him that he can "have Lena for Christmas".
Dad arrives in Lena's room wearing a dress and makeup and telling Lena that he is "Mum" before attempting to sexually assault her. Previous to this and unbeknown to Dad, Elbie had taken pity on Lena and left Lena's handcuffs loose allowing Lena to stab Dad with the weapon she had stolen. She flees downstairs, followed by Dad, who trips, causing Mum and Birdie to help him. Lena stabs Mum and Birdie attacks her, but Lena slams her into a wall and stabs her in the stomach.
She opens the back door, but Birdie attacks her again, so she hits her over the head with an iron she finds on the floor and rushes outside. She climbs over the back gate, but falls and twists her ankle. Mum and Dad rush out limping, bloodied and battered, chasing her into a field. Lena then falls and then Mum stabs her with a knife she took from the house. But Mum and Dad are too weak to kill her so Lena fights them off, and stabs them both repeatedly back and forth. Meanwhile, Elbie sets "the spastic" free of her restraints, before strangling her. He then walks out of the front door. The next and final shot is of Lena screaming in the field as planes take off overhead.

Question: What is wrong with Birdie's brother, Elbie?
Answer: Mute