Mon-El has a hard time of it in the Levitz, LaRocque, Lightle and DeCarlo story from Legion of Super-Heroes 23 (June 1986).
The issue opens with the Legionnaires reacting to Mon-El's reaction to Brainiac 5's news that he can do nothing to improve the anti-lead serum, and that Mon-El must re-enter the Phantom Zone, or die. By far the most significant thing on this page is Sensor Girl, who is shown hanging out in the Hall of Dead Legionnaires. Again, a really big clue to her identity, which I, again, did not see the importance to when I read this.
Cosmic Boy and Night Girl get a scene as they take a time bubble into the past for a vacation. This leads directly into the Cosmic Boy miniseries, a Legends tie-in.
I have to admit that, overall, this issue didn't really grab me. I loved the story on Talok VIII in which Mon-El had to relive his time in the Phantom Zone, but that wasn't so very long ago, and this feels a little like a rehash, even though the situation is different. Mon-El is forced into the Zone, and freaks out. Phantom Girl and Tellus enter it to find him and bring him back. I do like that Tellus finds the Zone, which everyone else feels is uncomfortably disorienting, a very pleasant and enjoyable place.
I also like the way the time bubble travels though the lines of colour to represent the time stream. The trip is more difficult than usual, but Element Lad, Dawnstar and Shadow Lass do succeed in getting back to Smallville, and bring Superboy to their period.
Jonah Hex makes a cameo in the story, seeing Superboy pilot the time bubble, in a scene that crosses over with the latest issue of Hex.
Brainiac 5 and Dream Girl use some of Superboy's blood to make a new serum for Mon-El, and this one is a permanent cure. Good thing, as we don't have to have this storyline again.