ians ▁dismiss ▁the ▁other ▁E tern als . ▁It ▁is ▁revealed ▁that ▁the ▁Doctor ▁won ▁the ▁race , ▁with ▁W rack ▁and ▁her ▁first ▁mate ▁having ▁suffered ▁" an ▁un fortun ate ▁accident ". ▁The ▁Doctor ▁ref uses ▁the ▁prize , ▁but ▁as ▁T url ough ▁helped ▁the ▁Doctor , ▁he ▁is ▁entitled ▁to ▁a ▁portion ▁of ▁the ▁prize . ▁The ▁Black ▁Guardian ▁rem inds ▁T url ough ▁of ▁their ▁b arg ain , ▁and ▁says ▁that ▁he ▁can ▁give ▁up ▁the ▁diam ond , ▁or ▁sacrifice ▁the ▁Doctor ▁to ▁gain ▁both ▁En light en ment ▁and ▁the ▁T ARD IS . ▁T url ough ▁h urls ▁the ▁diam ond ▁at ▁the ▁Black ▁Guardian , ▁who ▁van ishes ▁in ▁sc ream s ▁and ▁fl ames . ▁The ▁Doctor ▁points ▁out ▁that ▁En light en ment ▁was ▁not ▁the ▁diam ond , ▁but ▁the ▁choice ▁itself . ▁ ▁Cont inu ity ▁To ▁comm emor ate ▁the ▁show ' s ▁anni versary , ▁every ▁story ▁during ▁Season ▁ 2 0 ▁included ▁the ▁return ▁of ▁an ▁enemy ▁from ▁the ▁Doctor ' s ▁past . ▁During ▁this ▁tr il ogy ▁( beg un ▁in ▁the ▁serial ▁M aw d ry n ▁Und ead , ▁and ▁con cluding ▁with ▁En light en ment ), ▁the ▁enemy ▁was ▁the ▁Black ▁Guardian , ▁who ▁was ▁last ▁encountered ▁by ▁the ▁fourth ▁inc arn ation ▁of ▁the ▁Doctor ▁at ▁the ▁conclusion ▁of ▁The ▁Key ▁to ▁Time ▁s aga ▁in ▁the ▁ 1 9 7 9 ▁serial ▁The ▁Arm aged don ▁F actor