Santo in Anonymous Death Threat aka Santo en Anónimo mortal has a pre-credits sequence where an older man convinces an elderly jewelry shop owner to let him in after hours so he can buy a birthday present for his wife. Once inside, the client pulls a silenced pistol. “Is this a robbery?” the shopkeeper asks. “No, a murder,” is the reply, as the intruder fires.
Insp. Ponce and Det. Rocha of the police investigate. It wasn’t a robbery, since nothing was taken. The jeweler’s widow says she can’t think of anyone who would want to kill her husband.
Real estate agent Campos takes Fisher, a client, to the top of a tall building he is hoping to sell. “Up here you are safe from the smog,” Campos says, just before Fisher pushes him off the rooftop to his death. Campos’ partner cannot provide a clue; however, Rocha shows his supervisor an anonymous note Campos had received: “Your turn to die is next. You condemned yourself.” When this story gets into the papers, businessman Mario Gaos is upset: he also received a similar note, with a date fixed for his murder. His wife Ester wants to call the police, but Mario says “the only person who can save me…El Santo.”
After Santo’s match with El Polaco, Mario visits the silver-masked hero in his dressing room. Santo is accompanied by Yvette and Pablo, his crime-fighting associates. They go back to Mario’s house to discuss the case, and Santo says they will guard him day and night. As Santo departs, he is nearly the victim of a drive-by shooting. Meanwhile, Ponce and Rocha learn that the jeweler also received a death threat in the mail.
Yvette is stationed in Mario’s office to protect him. However, a window-cleaner enters through another room. Mario orders him to leave. Yvette and the man fight, but she is distracted when a bomb the man planted under Mario’s desk explodes. Mario is killed. The assassin knocks out Yvette and escapes. Later, discussing the case, Yvette asks if the killings could be political. No, Pablo replies, since the victims were all working in occupations far removed from politics. “Now,” Santo says, “but before?” Yvette researches the victims: the jeweler and Campos both emigrated to Mexico in 1945; Gaos came later, from Argentina, and Ester was his second wife.
Ponce and Rocha arrive and criticize Santo for not coming to them when Gaos asked for help. They agree to share their clues. Ponce gets a phone call: a man named Henkel says he got a death threat and barely missed being shot to death. Santo goes along with the police to the scene. Henkel says he is a machinery salesman and has no enemies. When Santo asks if he was in World War Two, Henkel stumbles around and finally says no. Later, Santo comes back and Henkel admits that he did oppose Hitler during WWII, even though he’s German. As Santo leaves, he meets Ester, who says she found some important documents in her husband’s papers. Santo agrees to visit her the next day, since he has a wrestling match that night.
Santo’s match against El Nazi is interrupted when a sniper shoots, missing Santo but killing his opponent. At her home, Ester gets a phone call: since the sniper failed, she is ordered to kill Santo the next day.
Santo is not impressed with the documents Ester shows him, and asks why she really wanted him to come to her house. She stalls, preparing the silver-masked man a drink. Fortunately, the phone rings: it is Ponce, who tells Santo that an autopsy proves Gaos was being slowly poisoned. Santo hangs up, and tells Ester he has to leave. “Did the Inspector discover something important?” she asks. “And at a very opportune moment,” Santo replies. “You haven’t had your drink,” Ester says. Santo: “Fortunately.” “Why?” Ester asks. “Oh…because I have to drive,” Santo says as he makes his exit.
Santo gets a call from Henkel, asking for a meeting in a warehouse. Naturally, it’s a trap and three Nazis attack. Santo bests them, but two escape as the police arrive. Insp. Ponce doesn’t understand why Nazis are suddenly showing up in Mexico, 25 years after the war. “In Europe, the United States, and some South American countries, Nazism has begun to flourish again,” Pablo suggests. They speculate that the victims may have done some special damage to Hitler and his followers during WWII. Henkel, when questioned, denies calling Santo. He admits that he gave testimony against a concentration camp commandant, Von Struber, who was sentenced to be hung. Another witness, coincidentally, is living in Mexico.
Dr. Fournier, the last witness, says he remembers the case. Von Struber “sent many people to the ovens.” He thinks Henkel died many years before. The same Nazi who blew up Gaos is sent to kill Fournier, but Pablo intervenes and shoots him. Meanwhile, Yvette has followed Henkel to a remote factory site in the countryside, and is contacted by radio. Santo decides to join her, but catches a Nazi planting a bomb in his car. However, the Nazi won’t talk and–to make sure–his confederates shoot him to death in a hallway of police headquarters!
Yvette is captured by Ester and taken to the Nazi hideout. She is forced to tell Santo over the radio that she’s OK, but she manages to tip him off to her plight and location. Santo discovers a secret entrance to the Nazi’s hideout in a rural cemetary, but is also captured. Pablo and Fournier, who were coming to warn Santo, are also imprisoned. Henkel is exposed as Von Struber, who wants revenge on those who testified against him at the war crimes trial. Ester is his daughter, who married Gaos only to further the Nazis’ revenge.
Santo and his friends are put in a cell, only “this isn’t a cell, it’s a gas chamber,” Santo tells them. However, they escape when two guards rather foolishly open the door and come in. Meanwhile, the police are closing in, and one of Von Struber’s associates wants to surrender. Von Struber would rather fight it out and then commit suicide, like Hitler did, so he shoots his cowardly underling. He tries to gun down Yvette, but only succeeds in killing Ester, his daughter. Eventually, he picks the worst possible hiding spot: his own gas chamber, which fills up with gas and kills him. Santo and the others escape and meet Insp. Ponce outside.