Superman travels to the future and meets up with the Legion when they're adults, to help them with someone attacking them who, gasp, seems to know all their secrets! Who could it be? No, it's not Superman paying them back for all the tricks they used to play on him. He takes out that repressed anger on Jimmy and Lois, remember?
The story mostly reveals which members will marry, or lose their powers, or whatever. So at least the cover was telling the truth, which I wasn't aware of since the copy I was reading has no cover.
The thing I learned, at least the thing I found amusing, was that as an adult, Brainiac 5 smokes a pipe. Not during battle*, but when the group is just standing around talking or planning, yeah, he's usually puffing away. As best I can tell, Curt Swan drew the issue, so maybe he just really wanted to draw Perry White and figured Brainiac 5 could be a green Perry White. Brainy has the receding hair to go with it.
Actually, most of the guy Legionnaires - active or retired - have receding hairlines. Except Colossal Boy, who can't actually change size anymore, but he did grow a beard, so he's got that going for him. Oh, and Bouncing Boy had really slimmed down. I wonder if that had something to do with not having his powers anymore?
* Which would be kind of cool. Brainiac 5, chilling behind his force field while he comes up with some ingenuous device to help, just puffing away, enjoying his fine space-tobacco. I'm sure it's engineered to clean your teeth, freshen your breath, and give your lung cillia a good scrubbing. The future is amazing!