▁Doctor , ▁inform ing ▁him ▁of ▁the ▁death s ▁of ▁his ▁friends . ▁She ▁fires ▁the ▁miss ile ▁at ▁the ▁plane . ▁ ▁Cont inu ity ▁Events ▁from ▁the ▁Four th ▁Doctor ▁serial ▁T error ▁of ▁the ▁Z yg ons ▁( 1 9 7 5 ) ▁are ▁brought ▁up ▁in ▁the ▁episode . ▁Kate ▁mentions ▁this ▁previous ▁Z ygon ▁attack ▁taking ▁place ▁in ▁either ▁the ▁ 1 9 7 0 s ▁or ▁ 1 9 8 0 s , ▁referring ▁to ▁the ▁UN IT ▁d ating ▁controvers y ▁regarding ▁in ▁which ▁de cade ▁the ▁Third ▁and ▁Four th ▁Do ctors ' ▁UN IT ▁stories ▁actually ▁took ▁place . ▁Kate ▁references ▁Harry ▁S ull ivan , ▁a ▁companion ▁to ▁the ▁Four th ▁Doctor ▁in ▁this ▁serial , ▁as ▁the ▁" nav al ▁sur geon " ▁that ▁helped ▁create ▁the ▁Z 6 7 ▁anti - Z ygon ▁n erve ▁gas . ▁Kate ▁states ▁the ▁gas ▁was ▁developed ▁in ▁secret ▁at ▁Port on ▁Down , ▁all ud ing ▁to ▁Harry ' s ▁location ▁as ▁reported ▁by ▁the ▁Brig ad ier ▁in ▁the ▁serial ▁M aw d ry n ▁Und ead ▁( 1 9 8 3 ). ▁ ▁Following ▁from ▁her ▁appearances ▁in ▁" The ▁Day ▁of ▁the ▁Doctor " ▁( 2 0 1 3 ) ▁and ▁" De ath ▁in ▁Heaven " ▁( 2 0 1 4 ), ▁both ▁Os good s ▁are ▁shown ▁we aring ▁cost ume ▁elements ▁of ▁the ▁Doctor ' s ▁earlier ▁inc arn ations . ▁In ▁addition ▁to ▁the ▁Four th ▁Doctor