The other three, less popular, main characters are described as being away on private ventures: Johnny giving a lecture in London, Long Tom experimenting on an electrical pesticide in Europe, and Renny building a hydro-electric plant in South Africa.

Investigating the deaths of other witnesses to the attack (a fisherman on the river and an aviator flying that day) leads Doc to Winthrop's Shipyards.

The Silver Death's-Heads steal some blueprints to hide their secrets while Doc runs into Winthrop's Secretary, Lorna Zane, and efficiency expert, Harry "Rapid" Pace (who has the annoying habit of repeating almost everything twice).

Harry leads the Doc to the home of Bedford Burgess Gardner (owner of Transatlantic Lines and Winthorp's chief rival) based on a comment from the gang overheard at the shipyard.

Again they run into the gang, and pick up Hugh McCoy, Gardner's Financial Relations Consultant (advising on the merger with Paine L. Winthrop's company) and Pace's rival for Lorna Zane's affections.

Based on a map of New York Harbor found at the club before it exploded and burned down, Doc and Monk go out onto the foggy river to investigate.

Ull, the leader of the Death's-Heads (but not the "big brain" behind their plans) sets a trap for Doc with Pat and Lorna as bait.

They make sure to leave clues, especially a trail of footprints in vaseline (from a jar "accidentally" broken in the store), to lead Doc to their hideout.

Doc, hanging on to the side of the submarine, sneaks aboard the ship, rescues everyone, recovers a lot of stolen goods and escapes in a lifeboat.

Ham's research showed that "Gardner" had made over $1 Billion in the past year by using the gang to kill off opponents and rivals under cover of random crimes.

From his hidden garage, he drives an "expensive but unostentacious [blue] roadster", carrying a radio tracking device and radio compass (as well as an automatic transmission and almost silent engine), and an armoured truck disguised as a "shabby laundry truck" complete with fake engine noise and artificially bad suspension.

From his collection stored in the Hidalgo Trading Company warehouse Doc uses a fast, silenced Motorboat, capable of self-righting itself after capsizing with a pop-up gun and tripod.

In this story the principle modifications are a mechanism to release a chemical similar to squid ink, external-viewing television screens and dial-based sonic devices allowing it to successfully navigate through New York harbor, and its traffic, while submerged.

Ham and Monk both have their normal equipment of Super Machine Pistols, firing tranquillising "Mercy Bullets," and bulletproof vests.

According to contemporary news accounts, this sequel had been filmed in the Lake Tahoe area simultaneously with the principal photography for the first Doc Savage.