The subversion of trust is a central theme of Steven Moffats two-part finale. When Kate Stewart appears like an avenging angel her triumph is short lived. The cybermen escape and the Doctors trust is damaged after he is drugged and dragged aboard a plane. Its interesting and rather sad that UNITs history with the Doctor leads them to believe he wont do everything in his power to save Earth without coercion. Kates brilliant scientific mind remains trapped within a cage of military cynicism. Her betrayal mirrors that of the Masters. Missy/the Master is the Doctors oldest friend, one he knew in childhood. It is evident there was once much love between them. Their story together is akin to losing someone to mental illness or addiction. Their treatment of Osgood is telling. The Doctor values intelligence and curiosity and is driven to teach and mentor. The Master is threatened by those same qualities and seeks to crush and destroy.