Juan Pablo Castel, a lonely painter, narrates from prison the story of how he murdered María Iribarne. He believes she is the only person who truly understood a small detail in one of his paintings, which leads him to think they share a unique emotional bond.
A relationship develops between them, but Castel quickly becomes jealous, possessive, and obsessed. His need for absolute certainty and emotional fusion causes him to misread María’s actions and suspect her of betrayal. As his paranoia grows, he becomes increasingly isolated and unstable.
Unable to tolerate doubt and emotional distance, he ultimately kills her. The novel is a psychological exploration of obsession, loneliness, and madness, where the “tunnel” symbolizes Castel’s closed, distorted inner world.