A concierge named Cesar, working in a Barcelona apartment building, finds himself on the roof, musing about his inability to attain happiness despite the seemingly good things in his life. He wakes up in the apartment of Clara, a woman he is fixated on, and begins his day in the lobby. Throughout his routine, he covertly entangles himself with other tenants, including Ursula, who is the daughter of another resident. In a questionable transaction, Ursula receives hush money from Cesar for reasons that unfold later.
Cesar makes a poignant visit to his ailing mother in the hospital, where their communication is stifled by her poor health. His interactions spark a deeper reflection as he discusses Clara with her. Another tenant, Veronica, asks for a favor regarding her dogs, giving Cesar specific instructions on their care. However, in a fleeting moment of rebellion, Cesar disregards Veronica's meticulous directions, feeding one of the dogs, Rocio, a piece of pie instead.
Throughout this unraveling narrative, Clara engages with many personal letters, carelessly tossing them aside. Meanwhile, Cesar's obsession grows sinister; he hides beneath her bed, planning to use chloroform on her to further meddle with her products and provoke discontent. The next morning, as he slithers out of her apartment, Ursula confronts him, demanding more from their unsteady agreement.
As chaos ensues in the building due to Cesar’s negligence—leading to dead plants and a hapless cleaner's son suffering consequences—he continues his duplicitous efforts against Clara. With multiple schemes leading to an infestation of insects in Clara’s apartment, her health and peace of mind deteriorate. Despite a short reprieve from her rashes, she discover more than she bargained for upon returning home: an overwhelming infestation that compels her to temporarily stay with her mother.
When the police start an investigation into the harassment Clara faces, Cesar cleverly casts blame away from himself, framing the cleaner's son. As Clara's life in turmoil advances, Cesar cleverly concocts further schemes, manipulating situations to remain undetected, albeit with increasing risk.
Things take a darker turn when Marcos, Clara's boyfriend, enters the picture, searching for clues to the disturbances plaguing Clara. Despite Cesar's deceptive skills, he becomes ensnared in his own trap after a harrowing encounter with Marcos ultimately results in tragic violence. The severity of his actions culminates in a chilling threat to Ursula, cementing Cesar's downward spiral into sinister behavior.
In the aftermath, Clara moves on and has a baby, leaving Cesar to reflect on his failures. He pens a letter to her, entreating her to think of him whenever she gazes at their child, calling it a strange form of happiness for himself. Cesar’s journey is steeped in twisted obsession and dark consequences, showing how the quest for happiness can lead to the destruction of others.
