Passing the Torch: Part 2 - S1-E19
Plot hole: In the first episode of series 4, we are told that the Power Chamber was directly below the Command Centre. In Hogday Afternoon, part 2, at the end of series 3, the Command Centre was completely destroyed - so how could the remains of the Command Centre be next door to the Power Chamber?
Answer: The Rangers are human. Blue Centurion is a robot/cyborg.