</s> ▁" The ▁Z ygon ▁In vas ion " ▁is ▁the ▁sevent h ▁episode ▁of ▁the ▁nin th ▁series ▁of ▁the ▁British ▁science ▁fiction ▁television ▁series ▁Doctor ▁Who . ▁It ▁was ▁first ▁broadcast ▁on ▁BBC ▁One ▁on ▁ 3 1 ▁October ▁ 2 0 1 5 . ▁It ▁is ▁written ▁by ▁Peter ▁Har ness ▁and ▁directed ▁by ▁Daniel ▁Net the im . ▁The ▁episode ▁is ▁the ▁first ▁episode ▁of ▁a ▁two - part ▁story , ▁the ▁second ▁part ▁being ▁" The ▁Z ygon ▁In version " ▁on ▁ 7 ▁November . ▁ ▁In ▁the ▁episode , ▁a ▁peace ▁treat y ▁agreed ▁between ▁humans ▁and ▁shapes h ifting ▁al iens ▁called ▁Z yg ons ▁in ▁the ▁ 5 0 th ▁anni versary ▁special ▁" The ▁Day ▁of ▁the ▁Doctor " ▁has ▁started ▁to ▁fail . ▁A ▁radical ised ▁spl inter ▁group ▁of ▁Z yg ons , ▁seeking ▁to ▁no ▁longer ▁hide ▁among ▁humans ▁but ▁be ▁themselves , ▁takes ▁over ▁Z ygon ▁High ▁Command ▁and ▁seek ▁to ▁in va de ▁present - day ▁Earth ▁and ▁neutral ise ▁the ▁organisation ▁UN IT . ▁The ▁al ien ▁time ▁trav eller ▁the ▁Tw elf th ▁Doctor ▁( Peter ▁Cap ald i ) ▁attempts ▁to ▁keep ▁the ▁peace . ▁ ▁Plot ▁Ever ▁since ▁the ▁War , ▁T enth , ▁and ▁Ele vent h ▁Do ctors ▁ens ured ▁the ▁creation ▁of ▁a ▁peace ▁treat y ▁between ▁humans ▁and ▁Z yg ons , ▁there ▁existed ▁two ▁versions ▁of ▁UN IT ▁scient ist ▁Os good : ▁one ▁human ▁and ▁a ▁Z ygon