Santo vs. Capulina aka Santo contra Capulina presents Capulina as a night watchman in a warehouse for an import/export firm, but as expected spends most of his time sleeping. Two crooks break in to steal a particular carton, but are surprised by Santo. A fight breaks out; Capulina sleeps through most of it, but finally wakes up and cheers Santo on. He tries to help out, and traps one of the “crooks” in a crate–but it’s really Santo! Both of the crooks get away.
The box they were trying to steal contains crockery which, when smashed, reveals diamonds inside. Santo and the police chief agree that there may be more in the warehouse, and decide to keep an eye on it. Meanwhile, Capulina has obtained a Santo mask in a toy store and walks down the street wearing it. The two crooks from the warehouse spot him, and think it’s the REAL Santo. “But he’s so fat,” one says. “Idiot,” his partner replies, “he’s probably wearing a bullet-proof vest.” They follow Capulina to an athletic field, where he is “training” a group of young boys, and think they’ve discovered Santo’s secret identity.
Santo, eating breakfast poolside at his palatial home, is startled to see a young woman in a bikini giving him the big eye. He pushes her into the pool, where she promptly explodes! She was a robot, constructed by a scientist in the employ of master criminal Cedric, who hates Santo.
At the gym, Santo catches a young woman hiding in his dressing room–she says she is a reporter who wants an interview and was trying to get a photo of him without his mask. In the gym, a new wrestler is defeating all of his opponents easily; Santo discovers that he is another robot.
That night at the warehouse, Santo asks Capulina to help him catch the crooks, and gives him a signal watch to call for help if he needs it. After Santo leaves, the two henchmen return to the warehouse to plant bombs and destroy the evidence, but are surprised by Capulina; thinking he is Santo, they flee. However, Cedric sends them back in, and they knock Capulina out and take him back to their hideout. The scientist makes a duplicate Capulina, but the rotund comedian makes his escape. Meanwhile, the chief of police has also been kidnaped and replaced with a robot double, which tries to kill Santo, but fails.
At the wrestling arena, the woman “reporter” is trying to vamp Santo in his dressing room when Capulina bursts in. He incoherently tries to explain what happened, but Santo brushes him off. A short time later, Capulina’s robot double gives Santo a false lead to lure him into a trap later that night. The real Capulina thinks the young woman is Santo’s girlfriend, and when she gives him a mini TV camera to get a shot of Santos sans mask, he agrees. The young woman is the daughter of the scientist working for Cedric: she is forced to aid the villain in order to save her father’s life, since he is a prisoner rather than a willing ally of the smuggler.
Capulina asks Santo if he ever takes off his mask, to eat, sleep, wash, etc. “Of course I do, I wasn’t born with it,” Santo replies. He agrees to take off his mask if Capulina will help him catch the criminals–but he’s wearing another, identical mask, underneath!
That night, the robot Capulina attacks Santo in the warehouse; with the aid of the real Capulina, the robot is subdued. Capulina, pretending to be the robot, allows himself to be taken back to Cedric’s hideout. The scientist, his daughter, and the captive police chief try to escape; Santo also arrives. There is a general brawl in which the crooks are finally all captured. The film ends with a scene on the athletic field: the lazy Capulina is using Cedric’s wheelchair as he forces his young friends to exercise!