Here's the plot summary as provided by our friends at IMDB.com:
“Crash Corrigan, a recent graduate of Annapolis, and Diana, a go-getting reporter, join Professor Norton for a search for the source of a string of earthquakes, Atlantis. They ride Prof. Norton’s rocket submarine searching the sea, and little Billy Norton, the professor’s son, stows away, of course. When they find Atlantis they are caught in a war between peaceful Atlanteans, note their white capes, and war-mongering Atlanteans, note their black capes. After many harrowing moments for Crash, Diana, Prof. Norton, and Billy, they barely get away with their lives when they escape a tower of Atlantis raised to the surface for the sole purpose of dominating or destroying the Earth (which one depends on the compliance of the upper world dwellers.)”
So it’s easy to see the similarities. Both franchises have a leading trinity of characters – the hero, the heroine, and the scientist – as well as a blazing rocketship that’s going to deliver them to some far off, exotic location wherein evil has its sights set on destroying our Big Blue Marble. In fact, the film’s Wikipedia.org citation suggests in its opening paragraph that Undersea was Republic’s direct response to Flash’s popularity at the box office, so I think it’s safe for us to conclude the property was always intended to stand shoulder-to-shoulder in film history as similar theatrical fare.
As I’ve not seen any box office reportage from the era regarding Undersea’s numbers, I have also read that this was only Republic’s second cinematic serial (it was a twelve-chapter affair delivering a total of 226 minutes). I also know that there were still producing serials into the 1950’s, so I’m willing to assume that their productions did well enough to fuel these efforts for, at least, two decades, though I do know that serials pretty much disappeared in that era as audiences were transitioning (of sorts) from the silver screen to the small screen. Even Republic’s Commando Cody: Sky Marshal Of The Universe underperformed in theaters but found new life on the Boob Tube. (Isn’t it interesting that in today’s market – the days of ten-episode limited streaming series – it would seem that the big budget serial has made a return to form?)
Folks wanting to know a bit more about Crash are encouraged to check out the film’s Wikipedia page as that’s the best source of info I’ve been able to find on it to date. I have seen a couple of low-priced versions available on DVD over on Amazon.com, so it appears as if the property is in public domain.