The final of 181 initially published Doc Savage novels has The Man of Bronze fighting the forces of Hell itself. Since Lester Dent (writing as usual under the house name of Kenneth Robeson) had no idea the series was about to end, it's a little strange he took the final bow of Doc and Co. into Lovecraft territory. Dent's writing is at it's usual crisp style- short and too the point with no verbiage wasted. However, there is quite a bit of information about yachting and sail boating at the beginning of the book, suggesting that Dent had done his homework.
The novel begins with the discovery of a castaway on an island off the coast of Maine. Psychiatrist Dr. Karl Linningen has been sailing his yacht, the Mary Too, on a extended vacation. On the Canadian island of Campobello he discovers Gilmore, a geologist who's been missing for months after disappearing in a cave. Gilmore is certain he's been in hell and is sure the rescue party is a group of demons sent to take him back. Dr. Karl decides to take him to the nearest shore. Naturally, Doc Savage just happens to be in the town were they moor.
This is a more subdued, economy Doc Savage. There is some mention of his sidekick John "Renny" Renwick, but he never makes an appearance. Ditto for "Long" Tom, the electrical engineer. "Monk" Mayfair and "Ham" Brooks are the only two who follow along for the ride.
When Doc makes it back to the yacht, Gilmore has vanished from a locked room. They break down the door to find the mysterious Mr. Wail. Wail is one of the strange and more interesting creations in the Savage universe. He's a short little chubby man who claims to have been a Jr. devil sent from hell to fetch Gilmore back. He provides the comedy relief which is normally Monk's job.
The final portion of the book has Doc and company traveling down a deep cavern, in pursuit of the bad guys. But Doc ends up in a subterranean lair filled with fantastic creatures right out of The Mountains of Madness. Are they fiends from the underworld or hallucinations from an unknown gas? Dent leaves the answer open.
You can't help but wonder where Dent was going with this story line. Did he plan another adventure using the same sub-terrain? Dent wasn't known for re-using characters in his stories. It's hard to believe he would take Doc Savage, super scientist detective, into fantasy land.
But we'll never know.