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The beginning of this volume actually sets up three ongoing mysteries for the Legion. Saturn Girl mentions two pieces of unfinished business: "the Time-Trapper, the scientific criminal who escaped into the future" and "the unsolved mystery of the vanishing world [...] swarm[ing] with monsters." Chameleon Boy adds a third, "the recent deluge of hardened space criminals reforming and surrendering." As far as I know, this is the first mention of the mystery planet and the reforming crooks, but the Time Trapper bedeviled the Legion multiple times in volume 3. The Time Trapper ends up being the only one of these elements to come up again; if multiple recurring plots were being set up, they didn't pay off within the next year despite Saturn Girl's intentions.

Not that intentions count for much. The Legion doesn't finally defeat the Time Trapper because of anything they do here (or any of the preparations they undertook in the previous volume), but because he decides to attack them by sending a minion with a de-aging weapon, from which they are saved by the most contrived of circumstances: the spray from the Fountain of 1,000 Chemicals had something in it that "must've neutralized the Legionnaires' age-regression at infancy." Is having such a thing at a fun fair even a good idea? If these rare chemicals can interfere with the operation of time devices, what are they doing to the bodies of passers-by? (This story also establishes that it's Mother's Day on one page and that it's Halloween-time six pages later. Either the Time Trapper is substantially messing around with time but no one mentions it, or Jerry Siegel is a forgetful writer. You decide which is more plausible.)

I guess you have to appreciate the effort, though. This volume also features the first multi-issue Legion stories I can recall: one about the evil Dynamo Boy taking over the Legion from within, with the help from the Legion of Super-Villains (this is the earliest of their appearances that I've read), and one about the mysterious crime lord Starfinger ("more dangerous than Goldfinger," one cover trumpets; the James Bond film would have come out about a year prior).

Like so many Legion stories of this era, they range from terrible to contrived to terrible and contrived. This volume has less dependence on Legion members behaving erratically (though you still have Lightning Lad pretending to be vengeance-obsessed for somewhat ill-conceived reasons), but still multiple stories where someone in a mask is dramatically revealed as someone else, and the reasoning doesn't stack up. Superboy says he knew it was not Ultra Boy because Ultra Boy can only use one superpower at a time, so he couldn't have both seen through the lead mask with x-ray vision and used other powers, and so he concludes the unknown "boy" must be Supergirl. But as acknowledged on the next page, Supergirl can't see through lead at all, which really undermines his supposed deduction. He should have disqualified her as the suspect too! Lucky for him that red kryptonite had this "weird side effect."

The best part of the book is probably the feeling that the Legion is an ongoing saga, where things and people can change. Lightning Lad loses an arm, and instead of being brushed aside, his mechanical arm comes up in multiple stories. In another story, a Hero of Lallor (introduced in volume 3) goes misanthrope and fights the Legion, only to end up dead, the fact that he'd actually appeared before as a (sort of) hero adding a little bit of pathos. In one story, two pairs of Legionnaires even get married and quit.

Though this of course turns out to be yet another overcomplicated ruse, it sets the stage for what's to come in the 1970s and '80s, where relationships would increasingly dominate the storytelling.