High school student Yousuke Yamamoto drifts through life without purpose or motivation, his world having lost its color following the sudden death of a close friend. On a freezing winter night after a petty shoplifting attempt, he encounters a girl named Eri Yukisaki waiting alone in a deserted plaza. Before he can speak to her, an abnormally tall figure wielding a roaring chainsaw descends from the sky amid falling snow and attacks her.

To Yousuke’s astonishment, Eri fights back with superhuman speed and precision, wielding knives and a katana as if she has done this a hundred times. The chainsaw-wielding man retreats after being wounded, but he does not die. Eri reveals she is a first-year student at a nearby school and explains that this mysterious enemy appears almost every night. She does not know his identity or origin, but she can sense where he will manifest. Since the night of her family’s funeral, when he first appeared, she has been granted extraordinary physical abilities that allow her to confront him. She believes defeating him is the only way she can ever find true happiness again.

For the first time in months, Yousuke feels something resembling excitement. He begs Eri to let him become her partner, arguing that he needs a purpose. Reluctantly, she agrees, though he proves largely useless in actual combat. Instead, he serves as her witness, her supporter, and the one who waits for her after each battle. Their nights together become a strange routine: studying in the library, riding his bicycle to the night’s battleground—which might be an abandoned swimming pool, a theme park, or a deserted village—and fighting a monster that cannot be permanently killed.

As Yousuke and Eri grow closer, he learns the weight she carries. Her parents and younger brother died in a car accident, leaving her completely alone in the world. The chainsaw man first appeared at their funeral, and every subsequent battle has forced her to relive her grief. Yousuke, in turn, struggles with the memory of his friend Noto, a boy who lived with fierce conviction before dying in a motorcycle accident. Noto’s voice still echoes in Yousuke’s mind, calling him a coward for avoiding anything real.

When Yousuke’s parents announce they are moving to Sapporo and he must transfer schools, he faces a painful choice. He tries to convince Eri to stop fighting, arguing that her enemy grows stronger whenever she feels genuine happiness—a cruel connection that forces her to choose between joy and survival. Eri refuses to run away, insisting that only by finishing this fight can she truly honor her family. Yousuke then hears Noto’s voice again, accusing him of having no resolve. At his farewell party, his friend Watanabe plays a song with lyrics written by Noto before his death, words that perfectly describe Yousuke’s relationship with Eri.

Realizing he cannot abandon her, Yousuke rushes to find Eri already facing the chainsaw man alone in a final, desperate battle. The true nature of the enemy is tied directly to her grief and her will to live. The fight is not merely physical but existential—a confrontation with despair itself. As snow falls around them, Yousuke must decide whether he is willing to risk everything for someone else’s sake, and Eri must decide whether happiness is worth the pain that always follows it. Their final battle forces them both to confront the meaning of survival when the world offers no guarantees.