This is a dated movie that you have to take in its historical context. A bunch of free-wheeling hippie girls share a flat, a carefree attitude to sex, and a shocking disregard for the physical integrity of their big-spending boyfriends, whom they have communally decided to off after a few days of TLC. This attitude of breaking with conventions, political ideology, availability and excessive violence describes the Zeitgeist of the late 1960ies and early 1970ies very accurately.
The balance is offset when an old boyfriend arrives on the scene. As he describes himself bluntly: "I've got this washed-out charme that's irresistible". He is a memorable and unique character, a freeloader, strangely hideous like a Mick Jagger stand-in and immature, and still captivating. When gang leader Peggy (played surprisingly well by the iconic Uschi Obermaier, who was actually more of a "media personality" than an actress) hesitates to comply with the group's five-day-rule she offsets the carefully balanced group dynamics. The movie finishes with a memorable showdown on beautiful lake Starnberg: "Are you hit?" -- "Just in the lung. No biggie." ("Du, nur'n kleiner Lungendurchschuss.")
The movie could have been better if there had been a few, homeopathically dosed scenes of ultragraphic violence. And as much as I liked the Thomas character (the movie is worth seeing for him alone), actor Marquard Bohm slurs his lines really badly.
But those are minor grievances, Rote Sonne is a bold, outstanding dystopic movie.