Storyarcs

Chainsaw Man opens on Denji, a teenage devil hunter drowning in the debt his dead father left to the yakuza. His only friend is Pochita, a cheerful little Chainsaw Devil with a pull cord in its chest. Denji sells body parts to stay alive and dreams of nothing grander than jam on toast and a real girlfriend, right up until the yakuza, under the control of a devil, betray him and hack him apart. Pochita merges with his heart and Denji rises again as Chainsaw Man, shredding the Zombie Devil and his killers in a spray of blood.

Public Safety Devil Hunter Makima finds him in the carnage. She offers him a choice: live as a human at her side, or be put down like the monster he now is. He chooses her, obviously, and the arc ends with Denji arriving in Tokyo and being dumped into Division 4 under his new partner Aki Hayakawa, who despises him on sight. It is a short arc, but it sets the core tension of the entire show. Denji is a dog on a leash chasing the crumbs of a normal life, and Makima is quietly reshaping him for purposes he cannot see.

Denji's first real job as a Public Safety hunter drags him into the orbit of Power, a Blood Fiend who volunteers a deal: rescue her kidnapped cat Meowy from the Bat Devil and she will let him touch her chest. Denji, being Denji, accepts without a second thought. Power of course betrays him the moment it is convenient, handing him to the Bat Devil to be eaten, but Denji hacks the devil in half anyway and actually rescues Meowy. It is somehow more than Power expected from anyone in her life.

The arc widens the cast quickly. Himeno, Aki's longtime partner with a Ghost Devil contract paid in her right eye, steps in during the Leech Devil ambush. Kobeni Higashiyama and Hirokazu Arai join the rookies on Division 4. And in the background, Makima finally spells out Denji's purpose: if he kills the Gun Devil, the monster who murdered 1.2 million people in five minutes, she will grant him one wish. It is the arc where Chainsaw Man stops being a lone survival story and becomes a team book.

Division 4 rolls into a quiet hotel to recover a fragment of the Gun Devil and walks straight into a trap. The Eternity Devil folds the eighth floor into an infinite loop and offers them a deal: surrender Denji and everyone else walks out alive. Kobeni panics and nearly stabs Denji to make the trade, Aki takes the blade instead, and Himeno tries to hold the line while Denji just jumps headfirst into the devil's body.

What follows is three full days of Denji tearing the Eternity Devil apart from the inside, running out of blood, eating the devil's flesh to refuel, and discovering that he can just drink his enemy and keep going forever. The monster begs for death before he is done. The arc hands Denji his first real power up, unlimited stamina if the fight has blood in it, and gives Himeno a quiet scene at the bar where she tells Aki the devil hunters she fears are the ones missing a few screws. It is a love letter to the three day fight, and a quiet warning that Denji is much further gone than anyone notices.

The season's long final arc begins with a kiss and ends with a rematch. Himeno takes an exhausted Denji to her apartment, seduces him, and gets her heart broken by a Chupa Chups lollipop. Then Katana Man and Akane Sawatari hit the city with the Snake Devil, wiping out Division 4 in a single coordinated strike. Himeno burns her entire body as the cost of the Ghost Devil's full power to save Aki. Hirokazu and Fushi die. Denji is captured. Makima is shot through the head on a train and calmly massacres every Gun Devil contractor in the car from across the room, using a long range power nobody will explain.

Aki wakes up in the hospital already broken, and Kurose and Tendo from Kyoto walk him into a new contract with the Future Devil. His right eye in exchange for seeing the 'best future,' whatever that turns out to mean. Kishibe, the scarred devil hunter from episode five, is pulled out of retirement to train Denji and Power through sparring that looks a lot like abuse. The raid on Katana Man's hideout becomes the season finale. Aki kills the Ghost Devil after it hands him a final cigarette from Himeno, captures Akane (who immediately kills herself via the Snake Devil), and Denji rematches Katana Man as Chainsaw Man in a straight head to head brawl. The arc ends mid fight, on the whir of chainsaw blades and the first glimpse of Reze in the closing credits, setting up everything Season 2 will spend its time unraveling.