PLOT
PART I — — Public Safety Arc
Denji is a young man trapped in poverty, working off his deceased father’s debt to the yakuza by working as a Devil Hunter, aided by Pochita, his canine companion and Chainsaw Devil. Denji is betrayed by the yakuza, who kill him for a contract with the Zombie Devil. Pochita makes a contract with Denji, merging with him as a human-devil Hybrid, under the condition that Denji live out his dreams of living a basic life such as having bread with jam, or falling in love with a woman and being intimate with one. By pulling on a cord in his chest, Denji then becomes Chainsaw Man and massacres the yakuza. In the aftermath, he joins a team of governmental Devil Hunters, the Public Safety Division, led by Makima, who assigns him to kill the Gun Devil. Denji fights through multiple enemies who seek Chainsaw Man’s power, and trains with his partner, Power the Blood Fiend, under the veteran Devil Hunter Kishibe.
Makima later turns out to be the Control Devil, ultimately seeking to control Pochita, as he has the power to erase evil concepts from the world by consuming their Devils. Makima almost defeats the Gun Devil, until it retreats inside the dead body of Denji’s partner, Aki Hayakawa, forcing Denji to kill him in self-defense. Later, she kills Power in front of Denji, causing him to turn catatonic and turn his body over to Pochita.
However, Kishibe organizes a raid to rescue Denji, and with his help as well as a frightened former teammate named Kobeni Higashiyama and a fragment of Power's blood, Denji kills Makima. Kishibe later reveals a reborn Control Devil, now a little girl named Nayuta. He asks Denji to look after her, and Pochita supports this mission in Denji's dreams. Denji begins attending high school while also fighting Devils as a vigilante.
PART II — — School Arc
Asa Mitaka is an introverted high school student attending East High School, who has trouble finding her place in society following her mother's death to the Typhoon Devil. One day, after accidentally killing her class' pet Chicken Devil Bucky, she is confronted by the Class President who has made a contract with the Justice Devil. She attempts to kill Asa, but Yoru, the War Devil appears in the form of a bird and makes a contract with Asa, inhabiting her body and making her a living Fiend host, under the condition that she kills Chainsaw Man. After her quick vengeance, Asa seeks to find Chainsaw Man, though she and Yoru have differing goals: Asa wants Yoru to leave her body, while Yoru wants to reclaim the Nuclear Weapons Devil which was consumed by Pochita. They join the school's Devil Hunter Club, where they are partnered with Hirofumi Yoshida (a former coworker of Denji who is revealed to be attending Fourth East) and Yuko, an outgoing student who befriends her.
Meanwhile, Denji has adapted to his new life as a normal high school student, also attending Fourth East, while using his newfound superhero fame to attract women. Hirofumi keeps tabs on him under an unknown mission. Knowing that Denji is still desperate for a girlfriend, Hirofumi introduces Denji to Asa, who he attempts to reveal to her that he is Chainsaw Man, but fails.
Asa visits Yuko and later discovers that she also formed a contract with the Justice Devil and plans on enacting revenge on everyone who bullied Asa. Using Yoru's devil abilities, Asa confronts Yuko, but accidentally kills her. At that moment, a stranger walks up to her and revives Yuko for "her little sister", turning her into a giant devil rampaging through the school. Denji attacks Yuko, but she escapes. The now-disfigured Yuko bids farewell to Asa and goes on the run for help, but a mysterious impostor posing as Chainsaw Man kills her.
Following the incident, Asa and Yoru bump into and meet Haruka Iseumi, the President of both the Devil Hunter's Club and the Student Council, who has taken interest in her and the stranger. Haruka tells Asa and Yoru that he is Chainsaw Man, showing them the dangling cord attached to his chest as proof. Believing him, Asa is then made by Yoru to go find someone she can kill and turn into a weapon. Spotting Denji to be the perfect target, she then goes on a date with him in order to kill him and turn him into a weapon. During the date, while she is alone, the stranger suddenly appears behind her and the stranger then reveals herself to be the Hunger/Famine Devil or "Fami", who then isolates her, bringing Yoru and herself outside and seemingly assists her in her quest to turn Denji into a weapon as the two, along with Hirofumi and Haruka, are trapped by the returning Eternity Devil in an aquarium. During this time period, Asa learns that Haruka is not the impostor, and is instead a big fan of Chainsaw Man. Asa kills the Eternity Devil, and Yoru attempts to turn Denji into a weapon. To Asa and Yoru's surprise, this fails.
Later, Denji takes Asa to his house, where he explains the rules there, including not to make out with him. Yoru immediately possesses Asa and makes out with Denji, causing an arriving Nayuta to use her Control Devil powers on her. This leaves Denji shocked as Nayuta tells Denji that she turned Yoru into a pet as she does not like Asa's "scent", thinking Yoru is trying to take Denji away from her, only agreeing to turn her back if Denji lets her have ice-cream everyday and he does not have intimate moments with Asa. Nayuta tells Denji she is going to erase Asa's and Yoru's memories leading from the date up to Nayuta using her powers on her, which he agrees with.
Later, outside the high school, Yoru tells Asa that Denji might not be into her saying they might've been stood up. Asa takes this as a positive, as she is thankful he "forgot" but she goes to add she desires human touch as she was confined with Denji for too long but negates her affection for Denji on hormones and that her time would be better used in solitude, studying. She is then approached by Hirofumi as he asks her if she could keep him company. Both talk in the Devil Hunter club room and Asa notes that Hirofumi is similar to her, seemingly hinting that he likes her, but instead, Hirofumi asks her to stay away from Denji. Nayuta notices that there is a scent of a particularly nasty Devil nearby, and encourages Denji to investigate as Chainsaw Man. As a depressed Asa is walking back home, a dead body falls in front of her, as multiple residents of her apartment building come outside.
