Santo and the Tigress in The Royal Eagle aka Santo y el Tigresa en el Aguila Real opens with Raimundo, a foreman on a hacienda owned by Irma Morales being given a telegram to send. The message is relayed to Santo on his car phone: Irma, the daughter of an old friend of Santo, needs his help in a matter of life or death. “Goodbye, vacation,” Santo’s sidekick Carlitos complains. At Irma’s hacienda, she explains that her brother died 3 months ago in a fall; since then, there have been two attempts on her life. She thinks her neighbours covet her land and want to get her out of the way.
That night, Santo, Irma and Carlitos go to a cockfight. Irma’s rival is Manuel Villafuerte, one of her neighbours. Manuel encourages his wife Paloma to engage in a singing “duel” with Irma: they alternate improvised verses, insulting each other. Irma’s rooster kills Manuel’s bird, earning her a large sum of money. However, outside the palenque, Irma and Santo are attacked by three thugs. The men are driven off by Santo’s wrestling prowess and the whip-wielding Irma.
The next day, Irma and Santo are horseback riding when her horse is struck with a poison dart. Santo finds a long strand of hair on a nearby bush, and later sends it off for analysis. At dinner, Santo thinks the wine smells odd: tested on a cat, it proves to have been poisoned. After everyone has gone to bed, someone enters Santo’s room and tries to stab him. After a fight, the assailant is exposed as Raimundo, who was jealous of Santo’s relations with Irma. No sooner has Raimundo been locked up than Santo hears a noise outside and goes to investigate. He’s attacked by a long-haired hunchback and a tall guy, also with long hair. Santo is knocked out, but Irma’s pet eagle swoops in and drives the hunchback and the giant away. The long-haired duo hides in their lair, in the cellars beneath the hacienda.
Santo wrestles in a tag-team match for charity. Back at the hacienda, they discover that Irma’s guard dogs have been killed.
Later, while Santo and Irma are out hunting, Carlitos gets lost and strays onto a neighbouring ranch. He’s about to be strung up as a cattle thief, when Irma shoots the rope and Santo defeats the men.
The hunchback puts a poisonous snake in Irma’s bed, but her eagle saves her. Later, a big flower pot falls and nearly crushes Santo and Irma. This hacienda is not a healthy place to visit, apparently. Finally, Santo finds the entrance to the hunchback’s underground lair: the long-haired hunchback is actually Alejandro, a bald hunchback who works on the hacienda. He and his giant associate attack Santo, get him dirty, and finally whack him with a shovel and leave him for dead.
Alejandro and the giant, after stuffing Irma’s eagle into a gunny sack, plan to kill Irma. Alejandro explains, at length: he is really her uncle Jorge, her father’s brother, who was the black sheep of the family. The giant is Irma’s half-brother, the son of her father and Felisa, a servant on the hacienda. Because he was “deformed”, this illegitimate offspring was locked up in the hacienda’s cellars for 20 years. Now, Jorge says that he will control the hacienda when Irma is dead. He starts to strangle Irma, who calls to her half-brother for help. For some reason, the giant decides he will help, and is shot dead by Jorge for changing sides. Before he dies, the giant squeezes Jorge to death. And then, when it’s all over, Santo comes in!
As the film ends, Santo and Carlitos promise to come back and visit Irma.