In a nutshell...
An alien being from the 30th Century has travelled back to the 20th to steal Clark Kent's glasses. He's used the Kryptonian lenses to fashion super-weapons with which he's threatening Earth in the Legion's time. Superboy and the LSH attack the alien's ship, but the weapons prove too much for them.
Superboy realises that the only thing that can defeat Kryptonian tech based weaponry is MORE Kryptonian based weaponry, so the Legion travels back to Krypton and take more lenses from Jor-El's lab. The device they build with the lenses defeats the menace, and they all live happily ever after.
The scene on the cover is a 'symbolic' one. The Legion sneak into the lab, so they never actually meet Jor-El and Lara, although they do meet baby Kal-El.
Thanks! That sounds ridiculously goofy and incredibly fun. I hope DC gets around to reprinting that era of the Legion soon.