It would seem to be a play based on the Greek myth of Andromeda and the sea monster. If the seller is to be believed it took place on March 5 & 6, 1980 at the Koenji Kaikan and was directed by a guy who now goes under the name Honomu Kitamori. Mizutani (if he indeed appeared - we only have the seller's word for it, and the name on the poster is written with different characters from the ones Mizutani normally uses) played two roles - Perseus and the wastrel son.
The final line on the poster reads, "Love and death, swords and magic pursue each other on the strand - a wicked operetta."
The main Japanese Rallizes fan site makes no mention of this event in its detailed chronology.