In the general setup the middle of a 7 part story would be a dramatic dip, Part 4 had proved to be possibly the best episode so far.
The Doctor is in danger, due to be shot, and countdown reaches zero, to what effect?....
Penetration Zero has had a huge effect, something's been unleashed and the staff run in fear. Huge explosions happen outside and steam fills the air. Stahlman locks down in the control room and pushes the staff into the green slime on the drill. Earthquakes and tremors are reported all over the country, all looks bleak. The Doctor asks for help to save 'his' world. The Brigade Leader demands to be taken back with him. Petra tries desperately to get power through to the TARDIS council. Stahlman talks to Petra on the intercom, and the barrier is lifted, releasing the Primords.....
This is just brilliant, it's so dark, the scene of Stahlman pushing the employees faces into the green slime is so nasty. The music is so eerie, it just fits so well, there's something to bleak about the whole thing.
I think the Primords are a great creation, a new storyline featuring them would work so well.
Brilliant, loses a point as the cliffhanger is less thrilling then last weeks. 9/10