They all escape in the submarine. The reptiles want the Master to revive the rest of their people, but to do so the Master needs more electronic spares from the naval base. He arranges for the reptiles to attack the base and the Doctor and Jo are recaptured, along with Hart. The Master wants the Doctor to help him revive the reptiles and the Doctor agrees to construct a sonar device, similar to a laser, to pinpoint their bases. The Doctor manages to create a diversion and Jo and Hart escape in a hovercraft.
Hart returns with reinforcements to try and defeat the reptiles.
The Master escapes out to sea in a small jet-boat and the Doctor follows. They are both recaptured by the reptiles and returned to the underwater base. The Master connects his device into the reptiles' power system, but the Doctor reverses the polarity of the neutron flow which will result in a massive explosion. At the same time Walker requests a nuclear strike on the underwater base. The reptile leader has the Doctor and the Master locked up, but they escape from the cell and head for the surface where they are picked up by a hovercraft just as the reptiles' base explodes.