DDD4 - Inferno 4

In Episode Three, the differences between the real world characters and their parallel world versions are made quite apparent. In Episode Four, the characters' similarities become visible. The Doctor attempts to make some headway in convincing Liz that deep beneath her (damn kinky and sexy) Section Leader exterior beats the same heart that led the Liz Shaw that he knows to become a scientist with an open mind. It's stated that both Lizes studied physics at university. One made the choice to become a scientist; one joined the army.

But did parallel world Liz join the army on her own accord? Or was she forced into it? Petra became a scientist on her own, but the project is a "scientific labour camp", as the Brigade Leader states at one point. If the workers are slaves, what of the scientists?

The key lies in Greg Sutton. Although initially Greg appears to be a more groomed and staid professional, draped in a fine suit, it's quickly apparent that he is the same rakish, anti-establishment persona as his real world counterpart. Petra mentions that if Greg straightened up and flew right, he would make an excellent "servant of the state". Greg is still alive purely because of his usefulness to the project, the exact reason why The Doctor has managed to stay his own execution after fixing the project computer.

The Doctor doesn't need to convince everyone around him of his intentions, as he realizes the Brigade Leader, Benton, and Petra have made their choices. However, he realizes that if he can convince Liz and Greg, they can convince others.