' s ▁scar f , ▁one ▁Os good ▁is ▁shown ▁with ▁a ▁bow ▁tie ▁similar ▁to ▁the ▁Ele vent h ▁Doctor ' s , ▁while ▁the ▁other ▁we ars ▁a ▁question ▁mark - decor ated ▁tank ▁top ▁similar ▁to ▁that ▁worn ▁by ▁the ▁Se vent h ▁Doctor . ▁Both ▁have ▁sh ir ts ▁with ▁question ▁marks ▁on ▁the ▁col lar ▁points , ▁a ▁common ▁element ▁of ▁the ▁Four th , ▁Fif th ▁and ▁Six th ▁Do ctors ' ▁out f its . ▁Os good ▁asks ▁the ▁Doctor ▁why ▁he ▁does ▁not ▁use ▁the ▁question ▁mark ▁mot if ▁anymore , ▁to ▁which ▁he ▁rep lies ▁that ▁he ▁still ▁does ▁– ▁on ▁his ▁under p ants . ▁ ▁A ▁portrait ▁of ▁the ▁First ▁Doctor ▁is ▁displayed ▁by ▁the ▁st airs ▁in ▁the ▁UN IT ▁safe ▁house . ▁ ▁The ▁Doctor ▁remarks ▁that ▁he ▁once ▁" sn og ged ▁a ▁Z ygon ". ▁The ▁T enth ▁Doctor ▁kiss ed ▁a ▁Z ygon ▁imp erson ating ▁Queen ▁Elizabeth ▁in ▁" The ▁Day ▁of ▁the ▁Doctor ". ▁ ▁The ▁Doctor ▁res umes ▁the ▁position ▁of ▁" Pres ident ▁of ▁the ▁World " ▁and ▁the ▁use ▁of ▁the ▁plane ▁afford ed ▁by ▁that ▁position , ▁both ▁mentioned ▁in ▁" De ath ▁in ▁Heaven ". ▁However , ▁when ▁he ▁introdu ces ▁himself ▁as ▁President ▁of ▁the ▁World , ▁he ▁is ▁told , ▁" yes , ▁we ▁know ▁who ▁you ▁are ." ▁This ▁is ▁continu ation ▁of ▁a ▁running ▁jo ke ▁from ▁earlier ▁in ▁the ▁series ; ▁the ▁Prime ▁Minister ▁Har