Mouse - Ending Explained

The ultimate reveal of Mouse is that the protagonist, Jung Ba-reum, is the 'Head Hunter's' real son and the 'Seven Deadly Sins' killer from the first half of the series. The baby swap occurred because the mothers hoped that raising their children in different environments would prevent them from becoming monsters. However, the OZ organization (a shadow government group) intentionally triggered Ba-reum's bloodlust to prove the need for the psychopath abortion law. The brain transplant in Episode 6 was actually performed by the Head Hunter himself, who used Sung Yo-han's brain (the innocent son of the nurse) to save Ba-reum. The ending reveals that Ba-reum's newfound 'humanity' is literally the presence of Yo-han's brain tissue, which allows him to feel the guilt of his own crimes. In the finale, Ba-reum kills his biological father in prison and dies shortly after, finally fulfilling the wish of his younger self to not be a 'monster' anymore.

Critics and audiences have discussed several interpretations of the ending. One common alternative reading is that Ba-reum's 'redemption' is a form of medical determinism rather than true free will—that he only became 'good' because of a physical part of someone else's brain, which raises questions about where the soul resides. Another interpretation focuses on the mouse metaphor, suggesting that even at the end, Ba-reum was just a piece of a larger political experiment, and his 'choice' to confess was exactly what the hidden powers needed to pass their abortion bill. Some viewers also see the ending as a reincarnation metaphor, where Ba-reum's death and the symbolic return of the 'mouse' in the epilogue suggest a never-ending cycle of genetic experimentation.