The titular Reaper is just an ordinary guy called Tyler Randall who returns from the dead, after he falls a few stories out of a hospital window, thanks to meteor rocks. So rather than going into a life of seclusion like any rational minded person would, he decides to go around and finish off all the elderly members of the town like some kind of vampire. Fortunately though, this episode had much more a backbone to it than just the Randall storyline, one which concerned the bond between father and son with Whitney and his dying father, Jonathan and Clark and Lex and Lionel.
Clark was so unexcited about his annual fishing trip with his father he just goes around complaining about it before deciding to just go on the trip to keep his father happy. Whitney, who once again came across as something of a selfish twit, didn't want to visit his dying father in hospital because they wouldn't be able to spend any quality time together.
These things might have worked better had Whitney actually realised that it was probably better he spent time with his father and Clark had just gone on the fishing trip without moaning.
And as usual Lex got the more interesting end of the plotline because his dysfunctional relationship with Lionel makes for much more interesting storytelling. And Lex tells Jonathan why he enjoys being friends with Clark so much. Its because he wants to be like Clark, in the worst possible way.
At the end of the day, the moral of Reaper is to spend more time with your father's. Unless you're a Luthor then try and separate yourself. But one can't help but think that Reaper should have been much more poignant than it actually turned out to be...