Santo in The Bermuda Mystery aka Santo en el Misterio en las Bermudas co-stars Blue Demon and Mil Mascaras begins with a mysterious beacon rises from the sea, and an airliner disappears. Then a strange storm suddenly comes up. The next day, Ramiro and a young friend go out on the dock to fish. The boy reels in a strange object: Santo’s mask, covered with seaweed! Ramiro says, years before, he was present when some strange things happened, like planes and ships disappearing…
After a wrestling match, Santo, Blue Demon, and Mil Máscaras are greeted by their fans, including Ramiro, an old acquaintance of Santo. Several attractive young women in bikinis try to make the acquaintance of Santo and the others at poolside , but the wrestlers have to leave to prepare for an important meeting. Meanwhile, another mysterious storm appears, and Ramiro is stranded at sea in his little boat. He’s nearly sharkbait when Santo, Blue Demon, and Mil Máscaras show up and save him.
At the meeting, the wrestlers are informed by a representative of the nation of Irania that his country is about to sign a treaty with theirs, but agents of another country plan to kidnap the Princess Soreida, who is going to sign the treaty. Santo knows Soreida, whose mother is from the United States. A fake Princess will attempt to decoy the enemy agents, and Soreida will pose as a visiting karate expert.
Mil Máscaras is attacked by three men who knock him out and steal his watch. He’s aided by one of the attractive young women they met at the pool, not knowing that she is one of the gang of foreign agents. She later introduces two friends to Santo and Blue Demon. Santo and Rina go back to his hotel room “to talk,” but Rina slips him a mickey. She then quizzes him about his mission to protect Soreida, and, as an afterthought, asks him what he thought of the three young women he met. He says he thought two were “hypocritical and perverse,” but Rina was sincere. She revives Santo and tells him that her father was a scientist who disappeared on a flight over the ocean nearby.
After a workout in a high school gym, Santo, Blue Demon and Mil Máscaras are attacked by the spy gang. Meanwhile, two men in shiny silver suits appear and kidnap Rina. They drive to the dock and escape on a cabin cruiser, with the wrestlers in pursuit. However, the boat suddenly disappears. Rina finds herself in a strange place inhabited by men in silver jumpsuits. The leader, who is reclining on a yellow chaise lounge, asks her why she was associating with the spies. She tells him that she infiltrated the gang to see if they knew anything about her missing father. The leader tells her she can never return to the “surface world,” and says “take her to be purified!”
Back on the surface, a yacht arrives with the fake Princess, who is the target of an assassin’s bullet, that instead kills one of her bodyguards. Soreida and two assistants are greeted at the airport by Santo and his friends. The head spy takes photos of Soreida at a karate demonstration, which he compares with a photo album of the “royal family of Irania,” and realizes that the supposed Princess is a double. It’s too late for the stand-in, however, as she is murdered by another spy in her hotel room. The head spy kills one of Soreida’s associates, then escapes by helicopter.
Later, the spies attack and shoot Soreida’s bodyguards. Soreida manages to hold the two remaining female spies at bay until she is rescued ; the prisoners reveal the location of the spies’ headquarters, and the three wrestlers go there. After a battle, they triumph. However , the head spy is still free, and he tries to kill Soreida, but she is saved at the last moment by Mil Máscaras.
Meanwhile, Rina has been purified, by wearing a bikini and standing in a water fountain. She is overjoyed to see her father: he was brought to the secret city by the scientists who created it. They have been choosing select surface people and bringing them down, since they are convinced the Earth is doomed. Rina admits she misses Santo.
The treaty is signed, and Soreida boards her yacht to go home. Santo, Blue Demon, and Mil Máscaras go along for the ride. But as the boat sails, the strange beacon rises from the sea…
“And they were never seen again,” Ramiro finishes. He adds: “The predictions of the apocalypse are coming true. The end of the world is near.” The film ends with footage of an atomic explosion.