Julio Salvador directed this film, which is now a classic in Barcelona's film noir. Emisora Films produced the movie. The protagonists are Conrado San Martín, Tomás Blanco, Casimiro Hurtado, Manuel de Juan, and Elena Espejo. The film was shot in many locations in Barcelona, such as Ronda Universitat, the former Frontón and Atracciones Apolo, the headquarters of La Vanguardia, the Post Office building, the former Antonio López square (now Correos square), Las Ramblas or Via Laietana. We review all the locations of P.O. Box 1001. Attention, if you have not seen it, there are spoilers.

A young man named Rafael is murdered in the middle of the street in front of the Barcelona Police Headquarters. Miguel and Marcial, from the Criminal Brigade, investigate the murder and find a copy of La Vanguardia in the dead man's room. In the newspaper, he has marked an ad requesting a manager for a chemical products company, asking for a large bail, and with the indication to write for more information to P.O. Box 1001. With this discovery, begin the police investigations that will lead to the killer's arrest.

The film begins with an introduction to the case with a voiceover over images of Barcelona. We see Paseo de Gracia and Catalunya square, Francesc Macià square, the Arc de Triomf with cars passing below, the statue of Columbus and the tram passing by, via Laietana, and Diagonal avenue, with its obelisk in the middle of the Cinco de Oros square. In the introduction, they explain that the film has filmed in actual settings, the same streets where this story took place.

A train arrives at Bellaterra station, and the postman picks up the mail. He brings a letter to young Rafael Quintana (Carlos Muñoz) and his father, who live on a farm in Bellaterra. The young man leaves for Barcelona as a result of this letter. We see Rafael as he gets off at a bus stop on Via Laietana, 54. When he gets off, he is shot dead, and the police investigation begins.

In the next scene, we see Miguel (Conrado San Martín) walking through the corridor of the superior police headquarters on Via Laietana, number 43, waiting for the Commissioner and Inspector Marcial Velasco (Manuel de Juan) to finish their meeting. It will be his first murder case. The Commissioner's office faces Via Laietana, and we can see the buildings on the other side of the street. Then they question the witnesses of the murder.

The police call to the Taxi Cooperative since the murderers were in one of these vehicles. And we see their offices. Later, they question the victim's father, who gives them the keys to go to the farm in Bellaterra. There they discover the room of the victim revolted and the newspaper with the advertisement in La Vanguardia that will take them to the post office box that gives the film its title. Later, they visit the headquarters of La Vanguardia (Pelai street, 28) and review the files. This clue takes them to Castillejos street, 243. We see how Miguel and Marcial, the two policemen, drive down Castillejos street until they reach where this address is.

In another scene, we see a tram passing in front of the post office building (Correos square, at that time Antonio López square). We also see the former Correos metro station. Miguel waits next to the subway station, while Inspector Velasco is inside the post office waiting to see who opens the post office box 1001. A boy sells newspapers next to Miguel, and he calls a girl passing by pretty. She goes up the stairs of the building and into the Post Office. Miguel also notices her because he likes her. Inspector Velasco sees her take a letter out of the box. Both policemen follow her, Miguel goes on foot and the inspector by car. Miguel boards the tram on Paseo de Colón with her, and the inspector rides alongside the tram. She gets off at La Rambla with Carme street. We can see the Virgen María de Belén Parish in the background. The girl is walking along La Rambla. She's about to put an envelope in the post box on La Rambla when the two police officers stop her.

The woman's name is Carmen (Elena Espejo), and she lives at the Untaunz pension in Pi square. The police escort her to her room to question her. She is a racket player at the Frontón Cataluña. She says that they pay her to collect the envelopes from the section, put them in another envelope and throw them in the mailbox on La Rambla, but she doesn't know anything else.

We see, again, the facade of the Post Office building with the passing traffic: cars, buses, and tram. The two policemen talk to the postmaster in his office. Later, Carmen walks down La Rambla to the mailbox and throws an envelope inside. The postman gets out of the car and collects the letters in a sack. The car pulls up at the post office and takes the bags to sort. We see the entire sorting process inside the building until they figure out where the letter is going. The two policemen and the postmaster discover the person responsible, Antonio Benítez (Tomás Blanco). One of the agents follows him when he leaves through the door of postal workers, and the other goes to his house. He lives in the Hotel Jardín in Sant Josep Oriol square.

Miguel is following the suspect. He walks behind him as he walks up the Rambla until he stops to have a beer at a bar/kiosk. That was the Canaletas bar. It was located at the beginning of La Rambla, and offered water from the same source. There, Antonio opens the envelope and immediately goes to see Carmen. He later returns to the hotel, where he meets another accomplice.

We see a scene with a large building with a dome. This building is the Telefónica building in Cataluña square, 16. Antonio puts on a conference there. The inspector pretends to talk on the phone. So, he can listen to the conversation. Antonio has an appointment with Julián at 10 am the next day. Julián is his boss and his accomplice. They will meet at Banco Zaragozano (Ronda Universitat, 22). A bank that has now disappeared. Now is an Adeslas office. The police are waiting for him there.

Later, when arrested, he explains what happened. We see again the Paseo de Colón and the tram going by. There goes the young Rafael to the supposed offices of the company he will work for. It's a fraud and it does not exist. The cops finally discover the killer's lair. It is in a tower on Bonaplata street, 60. This tower no longer exists, and there are now flats. We see how Inspector Velasco, Miguel, and two more police officers get into two police cars at the headquarters entrance on Via Laietana. They go to the tower and find the taxi parked at the door, and the taxi driver, Pascual González "El Miradas" (Casimiro Hurtado), explains what happened.

Julián met him at Paseo de Colón. We see him standing in front of the office door with the taxi. He watches as young Rafael walks out the door. He speeds through a square and boards a bus in front of a building and a statue. This building is the Llotja de Mar (Paseo de Isabel II, 1) next to the Post Office building. Julián gets into the taxi with the taxi driver, and they follow him until he gets off at the bus stop on Via Laietana. They shoot him there. Later we see how a Red Cross ambulance arrives at the tower on Bonaplata Street. There they find an incomplete photograph, and the police visit the photographer who took it. His studio is at 80 Caspe Street, they say. Through the window, we can see the tower of the Barcelona Cathedral.

In the end, we see how Carmen plays in the Frontón Cataluña, located in the Apolo Amusement Park that used to be at Nou de la Rambla, 113. Now is the Apolo concert hall there. We see the entrance, the interior of the park, the Frontón, and one of the attractions with stairs, mirrors, and obstacles where the final chase between the two police and the murderer takes place.