The plot from "Romance on the Rio Grande" is utterly ridiculous. It relies on one of the worst and most over-used clichés in film/TV history--the identical strangers myth!
The Cisco Kid (Cesar Romero) and his dopey sidekick, Gordito, come upon a stagecoach with a dead as well as a severely injured man aboard. The injured guy is the spitting image of Cisco and he'll probably soon die! And, when he finds a letter upon this man that tells Cisco he's the grandson of one of the biggest land owners in all of Mexico, Cisco decides to assume the guy's identity and make himself rich. What he doesn't suspect is that someone at the grandfather's hacienda wants to inherit the place himself....and is responsible for the attack on the stagecoach and is not above killing Cisco as well.
In addition to the plot relying on a terrible cliché, it also seems to go on and on and on--and MANY times it looks like it should resolve itself quickly but didn't. The eventual ending to the plotters is decent...but took too long considering how early Cisco sorted all this out. Still, despite the film's many logical flaws, it's fun and worth seeing if you are a fan.
By the way, the baddie is played by Ricardo Cortez. Don't let this guy's Latin good looks and name fool you, he really was born Jacob Krantz and was Jewish! Apparently, Cortez decided to pretend to be Hispanic instead of Jewish because of the idiot antisemitic idiots of the world.