I've never seen this but I want to. What was the point of all those rainy Saturday afternoons, late night Fridays and Wednesday night double bills watching BBC 2 if something like this can just slide on by unseen.
To paraphrase Wikipedia..... During a journey to an Uranus an alien presence briefly assumes control of the ship's crew. When they awaken they notice a long period of time has passed. Upon landing on Uranus, they find a forested land oddly like our own rather than the cold, bleak world they were expecting. The forest is surrounded by a mysterious barrier and when one of the crew pushes his arm through it, it is freezes. New features and forms begin to appear each time they are imagined by the crew and soon they discover that they have been the victims of a mind controlling, one-eyed brain living in a cave. The alien brain plans to possess the astronauts and have them take it back to Earth where it will implement a plan for global domination.
One assumes it doesn't, they don't and ultimately that the good guys win. I'll let you know if I ever get to see it.