Dragon Ball Z is the sequel to Dragon Ball that follows Goku's adult life, where he and his friends defend Earth from increasingly powerful alien threats. It explores the more serious side of science fiction and action, while also narrating the coming-of-age of his son, Gohan, who discovers his own hidden powers. The story encompasses the Saiyan, Frieza, Cell, and Majin Buu sagas, adopting a darker tone and focusing on themes such as perseverance, sacrifice, and friendship.