The 1934 original is one of Leslie McFarlane's lesser entries in the series, as it is one long series of truly incredible coincidences. A single bad guy is both the mastermind behind a huge (multi-thousand-dollar!) fraud involving stock in a fake oil drilling concern and the kidnapping of a random Mexican teenager whose well-to-do father had insulted the scoundrel by refusing his suit of his daughter Dolores. Said teen just happens to stow away on the Hardys' flight into Mexico to investigate the fraud. Mind you, the swarthy villain 'Pedro' has both the fraud proceeds in cash and a seemingly kidnapped state's witness to worry about; but he is more concerned with delivering the usual round of death threats to the Hardys and with haunting, for aims unfathomable, the hacienda of Dolores's familia, a good hundred miles from his HQ encampment. Oh, and the guy has an absolute fetish for burning or branding his insignia (the titular mark) into things (and sometimes people) pretty much everywhere he goes. Because /that/ couldn't ever come back to bite him.
Whilst Fenton is off yet again proving his incompetence as a private dick, our heroes cajole their hosts, father and son, to guide them to the area where the supposed drilling concern has been pimping [sic] oil. En route, at just the right desperate moment when They Shoot Horses, Don't They, a Yaqui Indian, conveniently named Yaqui, rides up out of nowhere—like a Magical Negro, only /indigenous-enslaved/ rather than /imported-enslaved/. He of course is responsible for shepherding the useless Hardy boys through the rest of the story.
When the boys—by way of getting themselves captured—inevitably stumble upon the missing witness (or rather 'witless'), Elmer Tremmer, he turns out to have absconded of his own free will because Pedro has convinced him the U.S. authorities intend to throw him in prison for his rôle in the fraud; thus he not only won't help the Hardys escape from Pedro's clutches, he means to ensure they stay captured.
Yep, he is stupid enough to believe that the egregiously obvious scam artist who he knows perpetrated the oil fraud is telling him the truth and the District Attorney is lying to him. Typical fucking American voter.
But having established that much, in the next chapter Tremmer is leading a revolt and accusing Pedro of keeping him prisoner because the latter fears what will happen if Tremmer goes home to Bayport USA. Wait, doesn't that mean he's changed his mind as to whom he believes? No, because the /very next thing he does/—after he has subverted and retained all but a few of Pedro's followers and is already across a fucking river from the villain—is to offer to bring Pedro the great detective Fenton Hardy 'in exchange for letting him go.'
Throw in any number of ridiculous if temporary escapes—because the cartoon parade of villains on the Batman TV show have /nothing/ on the splendiferous incompetence of Hardy villains... and omfg. The stupid is monstrous strong in this one.