The Doctor and Liz explore their surroundings, and discover that they're in the caldera of a dormant volcano. An alien spaceship is embedded in the ground, and a device powered by geothermal energy from the volcano is generating a force field that renders the island invisible to the outside world. The Doctor theorises that the device has caused the volcano to cool down and lapse into dormancy, which means that the ship must have been here for quite some time. He and Liz then hear gunfire and screaming from the jungle, and investigate to find that the rescue team from the Constitution has been caught in the middle of a battle between giant crabs and giant bats. The crewmen are being slaughtered until the Doctor arrives with his sonic screwdriver, which drives off both the bats and the crabs. The Doctor claims to be from a British expedition, and although wary of rivals, Grover is grateful for the Doctor's help. It is now obvious that the search for Amelia will have to wait until morning, and the Doctor and Liz accompany the team back to the Constitution. On the way back, they spot strange marks in the jungle earth, which appear to be the tracks of a miniature tank.