After nearly 20 years, Superboy’s series in Adventure comes to an end with issue 315 (Dec 63). Reprints of earlier stories would run as a backup feature in most issues of Adventure through 1964, and of course he would appear in many of the Legion of Super-Heroes stories. His own book was still running as well. Superboy would return as the cover feature in Adventure Comics in the late 70s.
The Legion is very excited about their new Universe Monitor, but it quickly proves itself not up to the task as alien invaders manipulate its feed, while they invade Earth.
The Substitute Legion, not having any such equipment in their cave, see the invasion and fight off the alien raiders, and the Legion finally come to learn of their existence.
Impressed, Saturn Girl decides to hold a contest, and reward the highest scoring Sub with membership in the Legion. Each are given tasks that their powers seem useless for, but each also figures out a way to succeed, except for Stone Boy. He gives up on his task, asking Saturn Girl to complete it, in order to protect some villagers who have come too close to the beast he was supposed to apprehend.
His selflessness scores the highest points, and Stone Boy is awarded Legion membership, but declines so that he can stay with his friends. A nice little tale by Edmond Hamilton, that scares us with the possibility that a person whose power is standing motionless might have become a Legionnaire.
Red kryptonite is the menace in Superboy’s final outing in Adventure. He uses a piece he was previously exposed to in order to stop a cyclotron exploding, but that causes it to be able to affect him again. Though where the reed k originally shrunk him, this time it makes him gigantic.
Primarily, this is a goofy little story, played for laughs with the giant Superboy. Lana Lang tries to use the situation to prove the missing Clark Kent is really the giant Superboy, but Colossal Boy helps him cover his identity. After the red k wears off, Clark appears before Lana while a disguised Colossal Boy holds up a collapsing bridge.