In January 2007, a sixteen chapter continuation titled 21st Century Boys (21世紀少年, Nijūisseiki Shōnen?) ran until July, and was gathered into two tankōbon. It tells the story of Kenji Endō and his friends, who notice a cult-leader known only as "Friend" is out to destroy the world, and it has something to do with their childhood memories. The series makes many references to a number of manga and anime from the 1960s-1970s, as well as to classic rock music, its title being taken from T.Rex's song "20th Century Boy".