High school student Uichi once had a happy and popular life, but now finds himself trapped in a living hell. He is the target of relentless and cruel bullying at school, while his home life offers no refuge as his family is deeply dysfunctional. His mother is unstable, his sister is a shut-in, and his father remains oblivious to the chaos. On the verge of despair, Uichi clings to only one thing: a childhood promise he made with his first love and close friend, Yotsuha, to become a veterinarian together. However, Yotsuha disappeared from his life years ago.

Just as Uichi reaches his breaking point, Yotsuha unexpectedly returns. She moves into the house next door and transfers to his school, seemingly as cheerful and attached to him as she was in the past. To Uichi, her sudden reappearance feels like finding a lucky four-leaf clover in a barren field, a symbol of hope and good fortune. But Yotsuha is not merely a kind-hearted girl revisiting a nostalgic past. Her bubbly and affectionate exterior hides a fierce, cunning, and ruthless determination. She quickly takes it upon herself to become Uichi’s protector, confronting his bullies and using extreme, calculated methods to dismantle their campaigns of torment, with her only rule being "thou shall not kill".

The narrative soon reveals a central conflict that elevates the story into a romantic thriller with supernatural elements. Yotsuha is a time traveler. In the original timeline, Uichi took his own life six months in the future due to the immense pressure he was under. Wracked with guilt and rage, Yotsuha made a wish upon a four-leaf clover for the power to make Uichi happy. This power manifests as an ability to rewind time, though a watch that keeps track of her loops ages with each use and is beginning to break down. Her mission is a desperate series of trial-and-error loops to "set right what once went wrong" and save him from his dark fate, a quest made more urgent by a serious heart condition that she is postponing surgery for until her goal is complete.

Notable narrative arcs in the series focus on Yotsuha systematically dismantling the various threats to Uichi's well-being. An early major target is Yamato Yae, a popular and handsome classmate who initially appears normal. It is revealed that Yae harbors an obsessive love for Yotsuha and is a primary instigator of the bullying campaign against Uichi, acting in a desperate attempt to eliminate his rival. The story also delves into Uichi's own family, with Yotsuha confronting the toxic dynamics in their home. Beyond Yae, the series introduces a more sinister and mysterious hidden mastermind, an antagonist who uses the symbol of the "Clover" and is seemingly pulling strings behind the scenes to orchestrate Uichi’s suffering for reasons yet to be fully unveiled. As the manga progresses, Uichi slowly begins to emerge from his shell, and a tender, wholesome romance develops with Yotsuha. However, this budding relationship is constantly shadowed by the escalating tension of the psychological thriller, as Yotsuha battles against bullies, family dysfunction, her own deteriorating health, the unknown "Clover," and the very fate she is trying to defy.