Santo in The Mystery of the Black Pearl aka Santo en el misterio de la perla negra opens with a man trying to escape from the police by running his truck over a cliff and jumping out. They think he’s dead, but he’s hiding and he manages to pass the diamonds he is carrying to Andrés and Julio. They have the gems attached to the hull of a ship leaving Spain for Mexico.
In Mexico, Wu Li is appearing in a nightclub. Her boyfriend is beaten up by several gangsters and warned to stay away from her. Wu Li ‘s boss says she has to go to Panama on a job for him.
Santo is briefed on the smugglers’ plan. However, the same gangsters that beat up Wu Li’s boyfriend have bugged his room, and they waylay him as he drives to the port of Veracruz. Santo beats them up in a field, getting his white sport coat all dirty; however, by the time he boards the diamond-laden ship in Veracruz, he’s managed to get it cleaned up. Also on the ship are Andrés, Julio, and Andrés’ girlfriend.
As Santo is searching their cabins, one of the original gangsters whacks him with a board. Santo wakes up on deck.
The ship docks in Panama. Santo is waylaid again, this time on the street, and tied to a wharf piling. The tide is rising and it looks like he’ll be drowned, but somebody swims by and cuts him loose.
Santo goes to the restaurant where he startles Andrés, Julio, and Mara, who thought he was crab bait by now. As he’s sitting down, a knife whizzes by his head. A note attached says to meet “Wu Ly” at a nightclub nearby; he manages to find it, even though her name is now spelled “Wu Li” on the poster outside. Wu Li does a fairly long dance number, then goes to her dressing room upstairs. Santo follows. Her dressing room is the size of a huge apartment and just as luxurious. Santo unwisely takes a sip of drugged tea and passes out. He wakes up the next day and Wu Li apologizes: she tells him she slipped him a mickey to keep him safe, or make him trust her. Meanwhile, the ship has departed for Cartagena, so Santo follows in a light plane.
The white-suited “Che” Dávila sends a man out to retrieve the diamonds from the hull. Several men pretend to attack Mara and then try to kill Santo when he “saves” her, but they fail. Julio shoots at Santo and misses. Santo pursues him through the streets of Cartagena; they run into an amusement park, and Julio is struck and killed by one of the rides.
Andrés and Dávila sell the diamonds to some guy. Santo is spying on the transaction, but is caught and thrown in a cell in a colonial-era fortress. When one of the guards comes in, Santo jumps him. Santo does a pretty good high-dive himself, and is picked up by a passing boat.
Dávila takes the money he got for the diamonds and tries to buy pearls from a native chief. He shoots the chief and steals the pearls, but is caught by Santo and the local police when his jeep overheats. However, they let him escape so he’ll lead them to Andrés and Mara. The police raid their meeting place, but Andrés and Mara escape, after Andrés kills Dávila.
Santo flies to Puerto Rico, the ship’s next stop. The police search Andrés and Mara at customs, but they’re clean. The pearls are back in the metal box attached to the hull of the ship. Wu Li turns up and tells Santo to meet her at the dock . They follow Andrés and Mara, who go out to the ship in a dinghy. Andrés swims over and gets the pearls, but when he hands them over, Mara shoots him with a speargun. Santo is left treading water in the middle of the bay when Wu Li takes off in their boat.
Wu Li and Mara are celebrating in their cabin on the ship: they were in cahoots all along, and have disposed of their male partners. However, there is a knock at the door: it’s Santo, with a bottle of champagne. He sends a telegram to his boss in Mexico, saying he’s taking a vacation cruise, with two lovely women who will soon be taking long vacations at the government’s expense. As they recline in deck chairs, he advises them: “Take advantage of the sun because a long time in the shade awaits you.”