Continuing from Klein's Story is...
Survival of the Fittest (2010) By: Jonathan Clements
Following their adventure in Kenya, the Doctor brings Klein to a far flung world to try and show her a bigger universe. They end up embroiled in a conflict between the Vrill, an insect species, and the Winterlack, a vicious and barbaric species. As the few surviving Vrill struggle for survival against the Winterlack, something else stalks their tunnels threatening to kill all in its path.
The ecology of the Vrill is the crowning achievement of this story. It is fleshed out and believable, working perfectly with the lesson that the Doctor is trying to teach. The way the Doctor reasons with his foes is very inside his character while providing Klein with some openings for questions of her own.
The Seventh Doctor is still trying to turn Klein from her Nazi ways. As the Nazi philosophy worked largely due to the closed off nature of the Third Reich, he presents her with the perspective of another planet. Here he sets himself up as the Vrill's authority and even gets Klein to connect with some Vrill by giving them names.
Klein continues to evolve as a character. She shows some real signs of having learned things from the Doctor, but has she learned enough?
The Winterlack hits a little closer to home than might be comfortable. They are brutal and ruthless, but they have their reasons to attack the Vrill.
The monster manages to be both cruel and sympathetic. He moves beyond being a random additional threat and highlights the destructive nature of the Vrill-Winterlack conflict.
The tunnels of the Vrill are compromised by gas and explosions. The surface is threatened by the Winterlack while something stalks the tunnels. The feeling of hopelessness pervades this story creating a great horror narrative for the first episode. The following episodes develop it as more of a war story.
In the End
There is a great twist at the end that leads into the The Architects of History.
An great Doctor Who tale even without the twist at the end. The Doctor must defend an innocent populous against a ruthless aggressor is a classic Who trope. Thus, the real meat of this story is in Klein, the most unreliable companion. Even Turlough, who started as the Doctor's enemy, never had an ideology so radically opposed to his. With Klein, the story gets to play the Doctor and his companion as possible opponents; something that will continue in future Klein stories. This one is another must listen!
Buy it here from Big Finish! Next Time: The Architects of History