Steve (John Forsythe) lives in a really crappy town. When his sister dies, he rightfully blames the city and you wonder why he even stays in the town. So, to teach the town a lesson, he starts a panic when he simply tosses a rock into the reservoir--and the backbiting jerks in the town start all sorts of rumors and they start to believe someone has poisoned their water. They aren't sure WHO did it, but they are somehow positive their water's been tampered with by someone. And, when a doctor tests the water and tells them it's fine...they then KNOW he's in on it!!
The film is a story about crazy paranoia and it's not surprising it came out during the Red Scare. It's a decent little parable about this...but it also is a bit heavy-handed and comes off as a bit preachy. Interesting, absolutely, but from from a must-see.