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. Diving from the boat, Doc discovers a submerged radio-buoy at a point marked on the map. Further investigation is cut short by a shell apparently fired by an unseen submarine. 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. A fake snitch calls Doc with some information but is cut off by a mock attack in another all-night drug store. 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 follows the trail using ultraviolet light (because Petroleum jelly glows under ultraviolet light) but approaches the hideout itself in disguise. He is able to penetrate the hideout and successfully avoid the traps set for him but does not manage to rescue Pat or Lorna (as they are not really there).