Let Me In

In March 1983, in Los Alamos, New Mexico, a disfigured man is taken to the hospital. An unnamed police detective (Elias Koteas) tries to question him about a recent murder. The detective is called to a phone and told that the man's daughter was just downstairs. While he is on the phone, the man jumps out of the window leaving behind a scrawled note that reads "I'm sorry Abby."
Two weeks earlier, Owen (Kodi Smit-McPhee), an unhappy and lonely 12-year-old boy who is neglected by his divorcing parents, sees a young girl named Abby (ChloÃ« Grace Moretz) and an older man, Thomas (Richard Jenkins), moving in next door and notices that she is barefoot despite the snow. Owen meets Abby one night and asks if she is cold upon seeing no footwear on her feet. Abby states that she doesn't get cold. Owen and Abby become close friends and start communicating by Morse code through the walls of their apartments. At school, a bully named Kenny (Dylan Minnette) and two of his friends Donald (Nicolai Dorian) and Mark (Jimmy "Jax" Pinchak) constantly terrorize Owen, who lies to his mother about it but tells Abby the truth. Abby encourages him to retaliate.
Thomas murders a man and collects the blood, but accidentally spills it. Hungry, Abby attacks a concerned neighbor named Jack (Chris Browning) to drink his blood, revealing that she is a vampire. On another night, Thomas hides in the back of a teenager's car, murders the passenger, and tries to flee, but crashes the car and becomes trapped inside. He pours concentrated sulfuric acid on his face, disfiguring himself to prevent his identification. He is taken to the hospital, bringing the story back to the first scene.
Abby learns what happened and climbs up the hospital building to see Thomas after speaking to the front desk worker who noticed that Abby is barefoot upon departure and couldn't find her afterwards. She knocks on the window and he points to his throat, unable to speak, and leans out of the window; Abby drinks his blood. Thomas passes out and falls to his death. That night, Owen is awakened by Abby who insists he verbally invite her in. She spends the night in his bed upon discarding her clothes and agrees to be his girlfriend. The next day, Owen's class goes ice-skating on the river. Kenny and his friends threaten to throw Owen into a hole in the ice. Emboldened by his relationship with Abby, Owen hits Kenny with a metal pole, splitting his left ear and causing him to scream in pain. At the same moment, several kids spot a body encased in the ice and begin screaming. It is the body of Jack who Thomas threw into the (then unfrozen) river. Along with the body is Thomas's bag.
Later, Owen sees Abby and hoping to make a blood pact with her, cuts his finger. Unable to control herself, Abby laps up a drop of Owen's blood revealing her vampire form. Not wanting to hurt Owen, the barefoot vampire flees and instead attacks a woman named Virginia (Sasha Barrese) upon jumping out of a tree. Virginia's boyfriend Larry (Dylan Kenin) witnesses the attack and calls an ambulance upon chasing her off. The next night, Abby admits to Owen that she is a vampire and that Thomas was not her father. Owen discovers an aged photo of her with Thomas as a young boy. As Thomas continued to age, he posed as Abby's father while she did not physically age past twelve.
In the hospital the next morning, a nurse goes into Virginia's unit to open the curtains and finds Virginia feasting on her own bloody wrists. She has been turned into a vampire by Abby. As sunlight enters the room, Virginia bursts into flames that quickly spread throughout the unit, killing them both and setting the entire floor on fire.
Owen sneaks out to spend the night at Abby's. The next morning, the detective forces his way into Abby's apartment and finds her asleep. He begins to uncover a window but is distracted by Owen. Abby wakes and attacks the detective to feed on his blood. The dying detective appeals to Owen for help, but Owen does not intervene. Abby tells Owen she will have to leave town. They share a kiss and he watches tearfully as she enters a taxi and leaves.
During Owen's swim class, Kenny, his sadistic older brother Jimmy (Brett DelBuono), and their friends, are seeking revenge for the injury Owen had inflicted to Kenny's ear. They ambush Owen and throw him into the pool. Jimmy says that if Owen can hold his breath underwater for three minutes, he will only be cut on the cheek, otherwise Jimmy will poke out one of Owen's eyes. Owen is forced underwater by Jimmy. After one minute, Kenny and his friends realize this is excessive and try telling Jimmy to stop, but he angrily refuses. But then an unseen assailant breaks through the skylight, attacking the bullies, and Owen sees torn-off limbs sinking into the pool all around him. Owen emerges from pool to catch his breath and then sees a pair of familiar bare feet come near him. He then looks up, faintly smiling to see an off-screen Abby.
Owen leaves town in broad daylight on a train, traveling with a large trunk. From inside, a message in Morse code is tapped out. Owen taps out a response and then looks out the window, quietly singing to himself "Eat some now, save some for later," the jingle of Now and Later candy, which Abby and Owen had shared in an earlier scene.

Question: How does Abby and Owen communicate?
Answer: By Morse code through the walls.