Tsugumi Project manga volume 6 features story and art by ippatu.

After finally reaching the island of Sado, Leon and company encountered the race known as the Sky People, who proved to be Tsugumi's estranged kin. The cunning Omoikane then directed them to a derelict facility in the Forbidden Zone, hinting at answers to their deepest questions about the TORATSUGUMI project and the mysteries of the past.

A search of the facility yielded both a potential cure for radiation sickness and research notes that may allow Leon and Doudou to complete their mission. Unfortunately, no sooner did they exit the facility than they were met by Omoikane's forces, setting off a desperate struggle. Which side will emerge victorious?