Now, now, now: before you haters go all off on me and threaten to pull my genre credentials, let me assure you that I bought the Universal Monsters' Legacy Collection back in 2017, so I do have every intention of sitting through all of the features and penning separate reviews. I believe I've seen it in my youth -- back in the days when all one had was broadcast television screenings to get familiar with that stuff -- and I've seen billions and billions (not really) of other pictures since that time ... so let's not make too much of it at this juncture.
However, I am aware that Son Of Frankenstein was the first time that character of Ygor/Igor was actually introduced into the wider Frankenstein mythos. (Interestingly enough, the film would also serve as the last time that star Boris Karloff would appear as the pivotal monster, but that's a discussion for another time and another place.) From what I've read about Lugosi's casting (from IMDB.com and other places), the veteran actor was cast kinda/sorta on-the-cheap; allegedly, his screen popularity had been dropping, so producers got him as a bargain. This didn't sit well with director Rowland V. Lee. He started shooting Son without a completed script, so he used each and every opportunity as it appeared in the production process to actually beef up Karloff's part, thus fleshing out (pun intended) a wholly new corner of the Frankenstein Movie Universe. Good on the man for giving a proven track record to swing for the fences once more in a storied career.
Technically, this was the third film in the entire Frankie series, but it varied just a bit from continuity issues with Bride Of Frankenstein (1935). In that picture, mastered the ability to speak with others, but this talent is completely gone from Son. The film offers no explanation for it, and I suspect this probably wasn't so much an oversight by screenwriter Wyllis Cooper as it was a necessity for the story as crafted.
And, lo and behold, Universal Pictures really went all-in with the inevitably release of Son ... as they scheduled it for its full U.S. theatrical exhibition on -- dumm dumm dumm -- Friday the 13th, 1939!
God bless those marketing geniuses!
Here's the plot summaryk as provided by the good people at IMDB.com:
"Returning to the ancestral castle long after the death of the monster, the son of Dr. Frankenstein meets a mad shepherd who is hiding the comatose creature. To clear the family name, he revives the creature and tries to rehabilitate him."