Shûji Terayama gets full-on weird. I'm not really surprised that the short format suits Terayama's style. I'm slowly going through the man's work and while the films I've seen all contain moments of greatness, they're never quite consistent enough for a high rating. The Grass Labyrinth seems less worried at providing some kind of coherent narrative, and that makes a big difference.
There's an underlying plot about an adolescent chasing the lyrics of a lullaby his mother used to sing to him, but it's merely an excuse for some surreal scenes that appear to symbolize the boy's transition from boy into man. My advice is to let the dream-like wonder swoop over you and to not worry too much about what it all means (unless you really love to dissect films).
The film is beautifully stylized, with some memorable compositions, strong use of color and a fitting, surprisingly modern soundtrack. The actors surrendered to their performances, the runtime is perfect and Terayama manages to keep it interesting throughout. Some parts feel just a little too dated for an even higher rating, but this is by far the best film I've seen from Terayama. Well recommended.