Toño [Juan Manuel Bernal] is pulled into an interrogation room by police agent Garduño [Damián Alcázar], as a murder case unsettles a quiet town. He begins to recount a few weeks earlier, when he and his wife, Ana [Elena Anaya], and their daughter decide to return to Toño’s hometown after years in Mexico City. The homecoming soon grows tense when Toño runs into Andrea [Patricia Llaca], a woman he has lusted after since adolescence and who is now married to Nicolás [Mario Iván Martínez], Toño’s old best friend from high school.
Soon, hidden desires flare into a passionate and dangerous liaison. The lovers hide from their spouses, helped by Toño’s brother [José María Yazpik], who runs a hotel in the town and whose blue room becomes the secret rendezvous for the pair (hence the film’s title, The Blue Room). Yet despite precautions, word leaks out, and the town begins to talk about the affair. Toño himself starts to wrestle with the possibility of ending the relationship, torn between guilt, loyalty, and the pull of the old friendship.
Meanwhile, Nicolás’s health deteriorates, and one night he dies. Many— including Nicolás’s mother Dora [Margarita Sanz]— suspect that Andrea has something to do with his death, hoping to free herself to be with Toño. Garduño presents Toño with a letter from Andrea to him that reads “Now it’s your turn,” a clue meant to tighten the suspicion on both lovers, though Toño denies any culpability and continues his account.
As Ana discovers the affair, a fight erupts, and Toño storms out. When he returns, he finds the police at his house—Ana has died, poisoned— and he is taken into custody. Garduño reveals that Nicolás left a sizable inheritance to Andrea, thus fueling suspicions of a conspiracy to murder both Ana and Nicolás to claim the money. He also receives a tip, supposedly anonymous, which deepens the doubt about their guilt.
But a late visit to Dora shifts the entire landscape. In her kitchen, Garduño uncovers evidence that Dora was the anonymous tipster and that she possessed boxes of the same poison used to kill Ana. Dora breaks down and confesses, explaining years of hurt and resentment: her husband never forgave her for an infidelity, and she fears Andrea will finally take control of the bakery and leave her destitute after Nicolás’s death.
With Dora’s confession, Toño and Andrea are cleared, and they are released. That same night, after they reunite in the blue room, Andrea shares a chilling truth with Toño: on the night Nicolás died, she deliberately closed his oxygen valve, a decision she made not only to end his suffering but to secure a future for them both, a revelation that leaves Toño stunned and forever changes what he thought he knew of love, loyalty, and motive.
