Featuring a cast of eleven youth actors, DNA opens with a teenage boy, Mark, informing a girl, Jan, that an as-yet-unnamed male they know is dead. Mark and Jan go to their friends Leah and Phil to deliver the news to them.
The action moves to the woods, where other members of the group of adolescent friends are discussing what to do about the death. John Tate, the group's ostensible leader, attempts to downplay the seriousness of the situation and deny responsibility. Lou, another boy, repeats the phrase, "He's dead," prompting John Tate to impose a ban on saying the word dead. Danny insists that he can't get wrapped up in the death because he plans to become a dentist. Richard, Cathy, and Brian arrive to join the discussion. John Tate becomes irate when Richard unwittingly says the banned word "dead." John Tate forces everyone to agree they must stick together and be in agreement about how to handle the situation. Brian, who is crying, wants to tell someone the truth.
Phil, Leah, Mark and Jan arrive. John Tate makes Mark share the story of how Adam, a boy they liked to harass and humiliate, died. Mark explains that they were trying to see how far they could push Adam to try to impress them. After making him eat leaves and let them hit him and put out cigarettes on his skin, the group made Adam climb a fence and walk across a metal grille set over a giant shaft. Drunk on vodka they made Adam steal, the teens threw stones at Adam until one hit him in the head and he fell into the shaft. That's why they believe he is dead.
The others ask Phil, who wasn't present for the incident, what they should do; they've heard he is very smart. Phil, having said nothing so far, outlines an elaborate plan to cover up the death. It involves breaking into Adam's house, stealing a piece of clothing without touching it, and then getting a random man on the street to handle it so his DNA is on the article. The plan involves Brian lying to the headmaster at school about a "fat postman" flashing his genitals in the forest. Phil invents the description of the flasher. They will also have to fake sets of footprints so it appears as though Adam encountered a heavy-set man in the woods before disappearing. Phil says the police will assume Adam was abducted, and, as long as everyone sticks to the story, no one will get in trouble.
After some time has passed, Leah and Phil sit in a field. Leah talks about how the grief "pouring into the school" is making everyone strangely happy. Meanwhile, she is disturbed by what they've participated in. She talks about killing her pet rodent Jerry for no reason. Mark and Jan arrive and say they need to talk.
In the woods, Danny and Lou have assembled with Phil and Leah to discuss how the police have arrested a suspect fitting Brian's description of the fat postman. Brian is hiding because he doesn't want to have to identify the man. Leah reminds the others that there is no man who abducted Adam—he never existed.
Richard and Cathy arrive. Cathy is excited after having seen the reporters surrounding the police station; she wants to be interviewed on television. The others learn that the police found DNA evidence on the jumper (sweater) belonging to Adam; the forensics led them to arrest the postal worker. Cathy reveals that she purposefully found a fat postman who fit Phil's made-up description and got him to handle the sweater. The group realizes they've unwittingly framed an innocent man. Phil forces Brian to go to the police to identify the arrested man, threatening to kill Brian if he doesn't.
Several weeks pass before Mark, Jan, Leah, and Phil learn that Adam has been discovered alive in the woods. Along with Lou, the friends meet Cathy and Brian in the forest. Brian, who is heavily medicated and has lost connection to reality, says he found Adam living in a hedge. Adam has dried blood on his head from the severe injury; he has extreme memory loss. Adam explains that he fell into the shaft and landed on leaves. Disoriented, he crawled through a tunnel, emerging in the forest having forgotten who he was.
Phil asks if he is happy in the forest. Adam says he is. Phil gets Brian to take Adam back to his hedge. Phil then takes advantage of Brian's compromised mental state by getting him to suffocate Adam with a bag (off stage). Cathy oversees the killing. Leah objects and insists that they have to bring Adam back home, but Phil insists they can't risk exposing the cover-up. Later, Leah and Phil sit in a field. When he puts his arm around her, she gets up and leaves.
Mark and Jan discuss how Leah has left town without notice. The action moves to Phil sitting with Richard in a field. Phil is silent as Richard pleads with him to return to being social; he asks how long Phil can stay up at the field. Richard remarks that John Tate has had a religious awakening, Danny can't look into an open mouth without worrying he's going to fall in, Brian has been incapacitated by even-heavier medication, Cathy rules the social hierarchy with sadistic violence and Lou is her best friend, while Jan and Mark have been shoplifting and selling stolen goods.
The play ends with Richard asking Phil again when he is going to come back. Phil continues staring into the distance.
