An eroge developed by elf, and the first game in the "-saku" series. It was originally released for the PC-98 in 1995, and later remade for Microsoft Windows platforms in 1999. All artwork for both versions were illustrated by Yokota Mamoru.
Unlike its successors, Isaku is a horror-themed escape game that follows Kogure Kenta as he attempts to escape an abandoned school building with serveral other students trapped with him. All the while, he must outwit the titular antagonist Itou Isaku, who will attempt to rape and torture the other students should they be caught.
