McCoy is a Gee Man
A gang of outlaws down in Mexico have kidnapped an engraver and are forcing him to create forged plates so that the can print their own money and flood the northern states with it. The counterfeit money soon comes to the attention of the FBI but despite their best G-Men trying to track down the source of the steady stream of phoney dough they have got nowhere. So Tim Hayes (Tim McCoy) is sent in undercover as a horse riding cowboy with the intention of uncovering who is behind the racket and infiltrating their gang to collect evidence.
I think "Phantom Ranger" is the first western I have come across which is actually set in the 1930s or at least that is when I presume it is set as we have mention of G-Men, we are taken to the Bureau of Investigation in Washington and all sorts of other things which would be typical of a 1930's crime story. But of course there is the horse riding Tim McCoy in his cowboy get up and packing a pistol which makes it both a 1930's crime story but also a traditional western which works through a recognizable formula.
So in a way this crossover of being a 1930s crime story / western is the movies USP and when you get over this curious set up what we get is as I mentioned a typical western. As such we see Tim masquerading as an outlaw to infiltrate the gang, he has a few squeaky bum moments, a few fights of various sorts and of course as this is a 1930s western there has to be a girl involved for the hero to win. The thing is that once you get over that curious crossover of types "Phantom Ranger" ends up entertaining but ordinary.
What this all boils down to is that "Phantom Ranger" is certainly entertaining and in some ways different to what you might be expecting from a Tim McCoy movie. But whilst interesting in an oddity sort of way once you get past its quirky genre crossover setup it becomes routine.