Jun is kidnapped and murdered. Shin-ae, in agony, joins a Christian congregation and finds peace. She decides to forgive the killer and visits him in prison. He calmly says God has already forgiven him and that he has found peace. She collapses, feeling her forgiveness was stolen. Her faith turns to rebellion: she mocks the congregation, tries to seduce an elder, and harms herself, and she is hospitalized. Jong-chan stays with her throughout.
On release she goes to a hairdresser and discovers that the killer's daughter is the stylist. She leaves in the middle of the haircut and cuts her own hair at home, with Jong-chan holding the mirror. The final shot shows sunlight on a patch of dirt, the hidden sunshine of the title. She has not been redeemed, but she survives through a small act of reclaiming herself.