The Doctor, Ben and Polly land on Pluto's moon, Charon, inside a force field dome. They find a dead body, and are accosted by the crew of a spaceship that lands nearby. Two of the crew, Spinks and Ray, think the Doctor and his friends killed the man, Spinks, but their leader, Professor Magellan, a robot, determines that Spinks committed suicide.
The Doctor and his friends learn that Magellan and his group are on Charon to harvest water, which is in great demand during this time period. Spinks came on ahead of the others to find another member of their crew, Ask, with whom they lost contact.
The Doctor, Ben and Polly help the explorers unload their ship, and a fissure in the ground is discovered. Magellan orders Ben and the Doctor to accompany him, while Polly stays with the others.
The fissure leads to a cavern, in which they find a control sphere surrounded by seven coffin-like boxes. Several are occupied by humanoid aliens, but one box contains a human — Ask. Magellan kills the aliens, and the Doctor and Ben bring the unconscious Ask back to the camp.
Magellan's mistrust of the Doctor and Ben increases when he sees that Ben is angry over the killings. He arranges for Spinks to share a tent with Ben and the Doctor and to keep an eye on them.
In the middle of the night, Ben wakes from a nightmare to see that both Spinks and the Doctor are gone. He wakes the others, then he heads for the TARDIS. On the way, he finds Spinks' mangled body. Back at the camp, Ask wakes and starts screaming, "It's free, it's free!" While Magellan sedates her, Ben and Polly sneak off to the cavern, though Ben thinks that the Doctor has left them here.
In the cavern, they find the Doctor in one of the boxes. Suddenly they are attacked by a black shadow, but Magellan, who with Ray has joined them, can't see anything. The Doctor, from the box, sends up a wall of water that temporarily blocks the creature, and urges the other four to join him. The five of them together control the creature enough to send it away.
As the TARDIS crew leaves Ray, Ask and Magellan to finish their work on Charon, the Doctor explains to his friends that the aliens were keeping the shadow creature in check, and when one of them died, they recruited Ask to help them. When Magellan killed the aliens, the creature was set free. The Doctor trapped it in the ice of Charon and removed some of its aggressive impulses.
- The Doctor's eyes are "soft chestnut brown".
- At this point in time, the Moon, Mars and several of Jupiter's moons have been colonised by humans.
- The TARDIS key is in the Doctor's shoe.
to be added
- Ben still isn't convinced that this Doctor is the Doctor. (TV: The Power of the Daleks)
- Other "living shadows" later encountered by the Doctor include the Elemental Shade, (TV: Love & Monsters) the Vashta Nerada (TV: Silence in the Library / Forest of the Dead) and the Shadow Kin. (TV: For Tonight We Might Die)