We begin Medusa with Jeff (played by George Hamilton), playboy heir to a massive fortune, laying dead on a bed in a boat with some lady. In disembodied voice form, he tells us he died three days ago and now he is going to review the mistakes he made in his life. Any movie that starts like this is pretty much destined to be terrible.
The first mistake Jeff feels like reviewing is crashing his sister Sarah's wedding (by the way, it turns out the woman he was laying in bed with was his sister). He's dressed as Elvis and is drunk and this is basically the start of Hamilton's embarrassing mugging and hammy acting. Jeff is supposed to be crazy and carefree and so Hamilton portrays this by basically doing totally random things mixed with horrible impersonations of celebrities, delivering lines of dialogue that completely fall flat. Every single thing this guys says and does is gonna make you want to punch him.
It turns out that Jeff and Sarah may or may have not been cut out from the latest will of their dead father, and since they need that money, something has to be done. Sarah is just used to living the high life, but Jeff owes money to a mobster, Angelo, who borrowed the money from the mob. Meanwhile, there is a hilarious and creepy police detective piecing this all together. He's this really hairy balding Greek guy with the kind of mustache that only grew back in the 70s. And he says things like, "I am like a spider, or a flame. I wait for my prey to come to me." And then he walks backwards, disappearing into the darkness. I think he might be Dracula.
So all this is happening and it's a little hard to follow along because first of all the film is so dark and it's hard to see who people are or what they are doing, and second of all you have the constant distraction of Jeff all over Greece for the entirety of the movie. At one point tries to pick up a girl by introducing his dog to her with this unbearably hot pickup line gem: "It's Part Cocker Spaniel and Part Poodle. Some Call It a Cockerpoo...I Call It a Spadoodle." It works.
Anyway, as it turns out, Jeff and Sarah have had or are having some kind of intimate relationship but the movie would be exactly the same if she was just some girl he used to date but then she got married to some guy, yet they are still in love. So obviously that was just thrown in to give the movie some kind of sleazy edge, which I guess works. But you already know how it ends (they die), since the movie opens with them dead, so what's the point?