From December 1935, this adventure was a collaboration between W. Ryerson Johnson and Lester Dent. Now this is just speculation, but it sure feels as if the opening and closing chapters (set on the island) were by Johnson, with Dent handling the middle part (scenes in NYC). Johnson is a fine writer, but perhaps his style is a little more dense and slower paced than Dent's, with more descriptive passages. The middle third roars along at a full gallop, climaxing in Doc jumping onto a flying autogyro to save his friends on the roof of a burning building.
The main plot seems strongly derived from the classic short story, "The Most Dangerous Game" (a terrific film version of this had been made in 1932 by the creators of KING KONG). Here we have an island near the Galapagos where trick lights cause many ships to wreck themselves so that the crews can be used, not to be hunted for sport as in the short story, but as slave labor digging pits in the ground for some obscure reason.
The main villain calls himself Count Alexander Ramadanoff, a huge muscular psychotic who dresses all in black and seems to thoroughly enjoy terrorizing and murdering people for his own thrills. Like Count Zaroff from "The Most Dangerous Game", he has a mustache and pointed goatee. He actually beats Renny in a straight fist-fight, which gives him confidence later on to tackle Doc head-on. The Count likes to play the grand piano before committing some atrocity.
There is also his brother back in Manhattan, the smaller and more subtly dangerous Boris Ranadanoff. Both exiles who left Russia before the revolution, the two siblings have their own agenda and Doc has his hands full dealing with these guys.
If anything, the island is packed a little bit too full with deadly threats. In addition to an army of nearly naked whip-cracking slave-drivers, there are huge iguanas that resemble Komodo dragons, big aggressive land crabs, sharks, herds of vicious wild pigs, and a couple of poisonous centipedes. To top it off, the volcano which has created the island is already starting to erupt when the story begins. What else could go wrong?! Well, there is the sinister 'thumb-hole death' where men abruptly drop dead with a deep hole in their temple....
All five aides are in action, and seem in character except for a few oddities. Pat Savage is described as having blue eyes a few pages after her introduction mentions her having golden eyes. And Doc here speaks to her in Mayan, while in 1942 she has just learned it from Monk. Aside from those details, everything seems canonical. Poor Habeas Corpus has a rough time this exploit, spending most of the story being pursued by wild peccary-like pigs.
Doc himself is at his peak here, especially during the battles in New York. Three times in a row, thugs trap him at gunpoint and he outwits them with hidden gimmicks despite the fact they know about his tricks. After the bronze man has been searched and handcuffed, a nearly hysterical gangster yells, "You could yank out his teeth, shave his head and pull out his nails and he'd still have enough chemicals hidden on him to blow up a battleship." Sure enough, when he goes in with a machete to kill our hero, Doc leaps up, free and dangerous.
Add Russian to the list of languages Doc can read and speak. (I count eleven so far.)
A creative touch is the trap Doc has set up at the door to his hangar at the Hidalgo Trading Company, involving a simple mechanical dummy which resembles Doc and draws gunfire. 'Robbie the robot' has had his face shot off four times already, allowing Doc to pounce on the would-be assassins. (You know, a literal science-fiction type Robot is one of the few menaces I don't recall Doc and his crew ever facing...)
Physically, Doc is in his prime, so energetic that he seems ready to explode. He is constantly in motion...running up and down stairs, bending iron bars enough to squeeze through, jumping and climbing, punching out half a dozen guys at a time. He leaps up onto the endgate of a speeding truck on the highway, then jumps off that onto a passing car, climbing in to take the wheel from the startled motorist. (That guy had a story to tell when he got to the office.)
Fans of the classic Doc Savage stories should get a big kick from this story, but be prepared to make slight adjustments for Ryerson Johnson's individual writing style. It would be interesting to read some of Johnson's Westerns which weren't tailored for this series.