Plot

The TARDIS lands in a petrified forest on an alien planet. Determined to explore, the Doctor leads his companions into the metal city, where they discover the danger of what will become his deadliest enemies, the mutant Daleks. (Originally broadcast in seven parts.)—Sarah Hadley

The Doctor suggests they clean up and go outside to explore. He asks Susan to check the radiation outside, and the meter shows normal. After she walks away, the meter slowly moves up into the danger range and a warning light begins to flash.

Outside the ship, they find themselves in a strange petrified forest. Barbara and Ian hang back a little. Barbara says she is reluctant to get too far from the TARDIS. Ian points out the TARDIS is useless without The Doctor, as he is the only one who can operate it. They find what appears to be a petrified alien creature, but upon examination The Doctor discovers that the creature is actually made out of metal. When they reach the edge of the forest they see a magnificent metallic city in the valley below. The Doctor is eager to explore it, but Ian doesn't want to take any chances. Since it is growing dark, they agree to go back to the ship and discuss it. On the way back, Susan stops to pick a flower. Suddenly she becomes aware that she is being watched. As she looks around the area, a hand reaches out and touches her. She runs to the ship screaming.

Aboard the TARDIS, Barbara tries to reassure Susan, who is angry that no one believes her. Ian watches The Doctor as he takes down notes from a series of computer displays. Ian asks what different controls are for and how the TARDIS works, but The Doctor is evasive in his answers. Barbara develops a headache, and they all get food cubes from the ship's food dispenser. As they eat, they hear a sound like someone knocking on the outside door of the ship. They rush to the monitor but see nothing. Barbara and Ian want to leave immediately, but The Doctor insists that he wants to explore the city. When Susan begs him, he reluctantly agrees and starts the take-off procedure. While no one is looking, he reaches under the control panel and fiddles with a small part. A moment later, the TARDIS starts screeching and stops. On investigation, The Doctor "discovers" that the end has come off of a fluid link and the mercury has drained out. Unfortunately, he says there is no mercury aboard the TARDIS and the ship won't work without the part, so they realize they will have to go to the city after all.

As they leave the ship the next morning they find a metal box full of vials containing some kind of liquid. The Doctor has Susan leave it in the ship so he can examine it later, and they head down to the city.

When they reach the city, everyone is feeling tired and weak. They decide to split up to look for the mercury and meet back at the entrance. As Barbara moves deeper and deeper into the city, she does not notice a stalk-like camera following her every move or that doors are closing behind her. When she tries to return to the others, she finds she is cut off from the route she took and starts looking for another. She eventually becomes trapped in an elevator, which takes her down into the depths of the planet.

Back at the entrance, the others meet up and begin to worry about Barbara. They decide to try to follow her.

The elevator reaches the bottom of the shaft and releases Barbara. She steps out into a corridor, then screams as something starts to move toward her.