The planet is ruled by the Eight-Legs or giant spiders, and their Queen is the supreme ruler. They govern using guards chosen from among the planet's Two-Leg (human) population and their own phenomenal mental powers, amplified by the blue stones of the planet. The Doctor arrives and makes contact with Arak, who explains that the Metebelians are the descendants of the crew of an Earth space ship, which crashed hundreds of years before. A spider on board found its way to the Blue Mountains where, through the effect of the crystals, its progeny grew larger and cleverer. The Doctor works out that a “negative” stone can absorb and reject the power of the blue crystals. He starts a revolt among the humans, now protected by these stones, but this is not effective. Far into the Blue Mountains the Doctor explores there and encounters the Great One, an enormous spider which controls the world of Metebelis and desires power over other domains too. She craves the crystal as it will complete a lattice of hundreds of similar crystals on her web and magnify her mental powers immensely. She knows the crystal is still on Earth and sends the Doctor there to get it for her. He flees back to Earth with Sarah, not knowing the Queen spider has now implanted itself in his companion's mind.