Gt doesn't count in this fic, so the last time we seen Frieza, along with Cell, Dr Gero, and others watching the final battle between Goku and Kid Buu. This story takes place 5 years after Buu's defeat. This is an alternate universe, so Pan was born the same year Buu was defeated. After Buu's resurection within Uub as a baby, the others divised their own plans for revenge. Dr Gero dismantled Android 19 and took some uneccessary parts off of Cell. He then compiled all the new parts and assembled into one new Android, called Android Zero.