The odd couple undercover cops go on vacation over a long weekend to do some fishing on a lake at Captain Dobie's cabin. Starsky hates the outdoors but Hutch loves it. Their familiar arguing reiterating their contrasting life philosophies comes out continually.
They shop in the nearby town and locals are unfriendly contrary to what they've heard. They barely get out and back to the cabin as the sheriff (Charles Napier - in between Jonathan Demme movies) tries to run them in. They show their badges and tell him they better be left alone. But you just know that won't happen because it wouldn't make for much of an episode.
That same night a mysterious looking group of people in blood red robes making weird sounds appears on the other side of the lake. What could it be? The Bohemian Grove Ritual? No, it is the satanic cult that has some kind of hold over the town inexplicably holding their sacrifice right where they know two big city cops are on vacation. That would be fine except the cult practices sacrificial offerings of humans.
They also can't resist vandalizing the cabin with occult symbolism written in blood on the front door whilst Starsky ad Hutch were asleep inside. They then leave a rattle snake in the refrigerator for Starsky to find.
I'd say it is utterly remarkable that these upstate California devil worshippers could be that arrogant and dumb provoking two cops whom they know are going to leave in a couple of days anyway.
The resolution to this particular mystery renders the events leading up to it very nearly redundant. In any event it also covers territory previously done in season 2, episode 14 "Bloodbath' when Starsky was kidnapped by an evil cult. I regard that episode as among the very best in the entire series. They don't exactly top it with this copy.