Yoon Cho-won (Cho Seung-woo) is a nineteen-year-old autistic man who lives with his mother Kyeong-sook (Kim Mi-sook) and his younger brother Jung-won (Baek Sung-hyun). Cho-won has a passion for running, and Kyeong-sook supports him and sets goals for him. Kyeong-sook wants Cho-won to run a marathon in less than three hours, but Cho-won struggles to pace himself over long distances. Kyeong-sook hires Jung-wook (Lee Ki-young), a former marathon champion who is now the PE teacher at Cho-won's autism school, to be his running coach.
Marathon contains examples of:
- Afraid of Needles: Cho-won is terrified of needles. Kyeong-sook sometimes uses the threat of shots to get him to do what she wants. After he passes out during his first marathon, he resists the paramedics' help because they will give him a shot.
- Apathetic Teacher: Jung-wook works as a PE teacher as community service for a DUI, and only agrees to teach Cho-won because the hours will count towards his punishment. At first he does as little as possible, ordering Cho-won to run endless laps while he takes naps or attends horse races and at one point skipping training to take him to a bathhouse. He does eventually start actually teaching Cho-won by riding a bicycle alongside him to show him how to pace himself.
- Because You Can Cope: Jung-won demands to know why Kyeong-sook hardly pays attention to anything except Cho-won. Kyeong-sook snaps, "You and Cho-won are different!"
- The Big Race: After Cho-won passes out during his first attempt at a marathon, he starts training towards the Chuncheon Marathon. The climax of the movie takes place during the marathon.
- Bitch Slap: Kyeong-sook slaps Jung-won after he's arrested for kicking out someone's side mirror in a fit of road rage.
- Character Tics: Cho-won flicks his fingers, flaps his arms, and makes trilling noises with his tongue, especially in moments of strong emotion. He speaks mostly in a high-pitched monotone.
- Daddy Didn't Show: Cho-won's divorced father is so busy that he rarely shows up for anything, not even Jung-won's birthday. He does manage to attend the Chuncheon Marathon.
- Fantasy-Forbidding Mother: After Kyeong-sook realizes that she's been pushing Cho-won to run to make herself feel better, she develops the opposite problem and declares that from now on he will not be running at all. Cho-won has to sneak off to attend the marathon. When Kyeong-sook realizes what he did, she follows him to try to stop him.
- Happy Rain: Jung-wook teaches Cho-won that rain is ideal weather for running. When it starts raining during the marathon, meaning he can start running as fast as he'd like instead of going slow, Cho-won hops for joy.
- The Kindness of Strangers: During the marathon, Cho-won becomes exhausted and sits down in the road. When a stranger hands him a choco pie, he finds the strength to keep going.
- Lens Flare: Seen in some scenes of Cho-won running, and in an early scene of him in the woods as a child.
- A Minor Kidroduction: The movie opens with Cho-won as a child who has frequent meltdowns and gets lost in the zoo at one point. After his father walks out on him, Kyeong-sook takes him into the woods to try to teach him the names of things.
- Parental Abandonment: When Cho-won was a child, Kyeong-sook intentionally lost him at the zoo, thinking she couldn't raise him properly. She soon came to regret it, and when she finds out that Cho-won remembers the incident, she thinks that he keeps pushing himself too hard at running because he's afraid she'll abandon him again.
- Product Placement: Cho-won is prominently shown wearing New Balance running clothes in many scenes. The end credits show that New Balance was one of the film's sponsors.
- Self-Harm: Cho-won has a scar on his hand from biting himself. He used to do it all the time, but he hardly ever does it since he started running.
- Sensory Overload: When Kyeong-sook and Jung-wook start yelling at each other, Cho-won crouches on the ground, covering his ears and trilling.
- Third-Person Person: Cho-won doesn't often refer to himself, but when he does it's almost always in the third person.