While Kinnikuman is on vacation with Mari-san's kindergarten class at Easter Island, the young wheelchair-bound Kouichi-kun laments being with an idiot like Kinnikuman, preferring to instead be with his favorite Seigi Choujin Buffaloman. Meanwhile, the other Seigi Choujins are also vacationing at ancient landmarks.

Seigi Choujin vs. Ancient Choujin is the third theatrical film based on the anime series Kinnikuman. It was released in Japan on March 16, 1985 alongside Gu-Gu Ganmo, Dengeki Sentai Changeman, and Tongari Boushi no Memoru. It is set after the Golden Mask Arc.

This film is the first to feature anime original character Kinkotsu-Obaba. Although this film is set after the Golden Mask Arc, the character Geronimo does not appear as an active fighter until the next film. He does, however, appear in flashbacks to the Golden Mask conclusion. This is also the second time voice actor Ichirou Nagai appears in a Kinnikuman work playing the main villain. He had previously appeared in the TV Special. It also features the return of voice actors Kouzou Shioya and Yasuo Tanaka, who had previously appeared in the first film Stolen Championship Belt.