The protagonist is introduced as a 27-year-old software engineer living with his younger sister after their parents' deaths. He works overtime to support them, but his sister's romantic obsession with him grows increasingly overt. A flashback to one month earlier establishes their normal sibling dynamic, which is disrupted by a series of escalating physical and emotional transgressions. The story's central conflict emerges when the sister drugs him and initiates a sexual encounter, forcing him to confront her possessiveness and his own repressed desires. A fake boyfriend deal and a subsequent confrontation over her role in their parents' deaths propel the narrative toward a climax. The protagonist must finally decide whether to permanently reject her or embrace a life of socially forbidden partnership. The mid-game pivot occurs when he reveals that he changed their dying mother's last words, and the sister confesses she provoked the fatal accident, creating a shared moral equivalence.

Triggered by accepting her drugged tea and choosing to "Give In" during the first sexual encounter. This path escalates intimacy, leading to her confession that she caused their parents' fight and a mutual acceptance of their shared guilt. The protagonist eventually proposes marriage, and they move to a new city to live as a couple. In the final version, two endings branch from here: Together in the Darkness (romantic, sex-positive ending) and End of Obsession (the protagonist reveals he altered their mother's last words, acknowledging his own darkness). These endings are narrative alternatives, not sequential steps.

Unlocked by refusing the drugged tea. The protagonist firmly rejects her advances, and she accepts a platonic family relationship. They remain siblings, though she later reveals she lied about the internship. This ending is a dead end; an alternative version, Normal Family Continue, subverts it by having the sister declare the relationship as inevitable.

Achieved by selecting the "Don't drink" option, leading to a short conclusion where the sister accepts the rejection. This path is truncated.

A minor branch triggered by agreeing to a dinner date with Susan. The protagonist clarifies he is not interested in restarting their relationship, closing this subplot. This choice influences Lucy's reactions but does not alter the main route.