I read most of this series when I was a teenager. It's very cheesy, 80s sci-fi, fun to read, but not great literature and certainly not very original; indeed, it's more like a pulp adventure that resembled a TV series more than anything else. Dumarest is your typical super-competent action hero who has somehow been stranded far from Earth. He spends the entire series trying to get home, while being pursued by the Cyclan, a religious sect that acts as the recurring bad guys. The Cyclan are trying to breed emotion out of the human race, and Dumarest holds some secret they want.
I don't remember if it was ever explained exactly what it was they wanted from Dumarest, nor do I know if he ever made it back to Terra, but after you've read the first few books, they all become very much the same: Dumarest arrives on a new planet, runs into trouble with the locals, finds out there's some Evil Secret Cyclan Plan behind it, kills a bunch of people, and moves on to the next planet, still trying to find a way back to Terra. I've just described the plot of every Dumarest novel I read.