I suppose its a spoiler to tell you that the hunchback has a short-range teleportation device, but if you haven't figured that out for yourself early in the story, then you aren't really trying. But that doesn't mean there isn't a strong element of mystery to the story. The identity of the hunchback and his motivation are all uncertain at first. It seems like the hunchback might be a good guy of sorts--he claims he wants the convicts to help him destroy the syndicate that framed all of them. But he's also willing to commit murder and use the threat of death to keep his gang in line. It's soon apparent that--whatever his motivation--he's not on the side of the angels. (I keep saying "him," but that's just simpler than saying him/her over and over again.)
Doc gradually sorts things out, though at one point he's framed for murder and on the run from the cops himself. But, as is typical of the best Doc Savage stories, when the villain thinks he is one step ahead of Doc, it always turns out that Doc is actually two steps ahead of him.