The Acid Eaters (1968)

Director: Byron Mabe
Also Known As: The Acid People
Tagline: A film of anti-social significance!… An adult happening in psychedelic color!… The first Hollywood underground movie!
Plot: Remember the Drew Carey Show title sequence set to The Vogues' "5 O'Clock World," in which Carey and his associates danced their way through a typical work day? Imagine that bit extended to 10 times its length, and the catchy music replaced by close-ups of flushing toilets and people chewing sandwiches. Then imagine that at the end of the montage, a half-dozen or so of the cubicle drones meet up in an alley, strap on their biker gear, and drive to the woods to get loaded and murder squares. That's more or less the setup for The Acid Eaters, a late-'60s drugsploitation oddity that combines wanton violence and baggy-pants farce.
The bikers meet up by a lake, at a dock sporting a sign that reads, "Taking a trip? Go LSD… the only way to fly!" When they arrive, one of their members is already making it with his old lady underwater, emerging from the deep to gasp, "Welcome to the Submarine Club! You passed the test with flying colors!" There follows a long sequence of topless dancing and body-painting, then some lascivious rolling around in the grass, and then, inevitably, the slaughter of a passing motorist for pot money. (The gang's resident artist hangs a sign around the victim's neck, reading: "Here lies a man who lost his [drawing of donkey] so we could buy some grass.")
Finally, the bikers reach their ultimate destination: A massive pyramid made out of LSD-tinted sugar bricks, where they dance toplessly some more, flashback to traumatic childhood memories—including a disturbing scene where one of the women moans, "Daddy, let's not play hide-and-seek. Let's play the same game we played in the woodshed when I was 15. Remember?"—and are tormented by a demonic presence who looks like the Underwood Devil.