(Ben approaches the Cybermen.) (He then turns to the Doctor.) It's the deepest in the base. They're going to use the Z-bomb to destroy the Earth!
Plan: A: the base; Q: What do the Cybermen take over? A: the Z-Bomb; Q: What do the Cybermen want to stop the humans from using? A: the Doctor; Q: Who realizes the Cybermen are going to use the Z-Bomb to destroy Earth? Summary: The Cybermen take over the base again, apparently to stop the humans using the Z-Bomb, but then the Doctor realises they intend to use it themselves to destroy Earth.