Masked Rider Black: Hurry to Onigashima

Released on March 12, 1988 (between episodes 22 and 23) as part of the Toei Manga Matsuri film festival. Originally known simply as Kamen Rider Black during its theatrical release. Children all over Tokyo mysteriously disappear without warning. Kotaro suspects that the Gorgom are behind the abductions and follows a suspicious-looking tour bus with children on it, only to lose its trail off a cliff. With the help of a fisherman (played by Ishinomori himself), Kotaro travels to a remote island and (wikipedia)