The plot is the same as any of the plots used throughout Hokuto no Ken. Bad guys enslave innocent villagers, innocent villagers try to find peace and get slaughtered, Kenshiro avenges them by painting the walls with the innards of the baddies. I watched all the episodes in the original series, and it really was just that over and over again, just with a new, more powerful main villain to replace the last.
I think the only real reason people watch Hokuto no Ken is to see people get axed in the back, showered with arrows, slashed apart, and to see heads explode... And I suppose you'll find that here. Senseless, excessive violence with a weak plot.
I don't know exactly how long it's been since the world ended (or if it was mentioned, I forgot), but it's probably been at least 10 years judging by Lynn's age... Kenshiro's name spread across the lands and instilled fear in gangs everywhere. He defeated every great power. I really don't get why they'd still dare try to mess with peace when they're guaranteed certain death. Really, he defeated the baddest of bad in Hokuto no Ken 2. I suppose what these OVAs are saying is that the series will never have a true ending.
Well, it's kind of entertaining to watch. I suppose you'll have a hard time finding other anime with this kind of violence. Watch Hellsing Ultimate - it has better animation, story, more intense fight scenes, and more blood, but less guts.
