Metebelis III was a famous blue planet in the Acteon Galaxy. The Third Doctor had attempted to reach Metebelis III several times during his tenure as UNIT's scientific advisor on 20th century Earth, finally succeeding and meeting some of the dangerous native wildlife.
The blue moon of Metebelis III gave the whole planet a blue tint at night. It had many different habitats, including desert plains, green forests and mountain ranges. Native species included a giant snake, a species of giant bird and an intelligent, if primitive, species. (TV: The Green Death) It was later colonised by humans. (TV: Planet of the Spiders)
The blue crystals of Metebelis III could break hypnotic spells and mind control. The Doctor used the crystal to break the hypnosis of the computer called BOSS. (TV: The Green Death) They could also enhance intelligence and psychic powers. Another type of stone on the planet could counteract the energy produced by the blue crystals. (TV: Planet of the Spiders)
The Doctor attempted to visit Metebelis III a number of times, including once with Jo Grant, on his first trip with the TARDIS after his exile was lifted.
At some time in the planet's far future, a human starship crash landed there. Humans and sheep survived the trip, as did some spiders. In four hundred and thirty three years, the crystals mutated the spiders into larger, sentient creatures that called themselves the Eight Legs. They enslaved the humans, harvesting and devouring them like livestock.
The Eight Legs planned to take over the rest of the universe, but required all of the blue crystals. The Doctor's theft of one of the crystals caused the ruler of the Eight Legs, the Great One, to send her minions to Earth to retrieve it. The series of events which followed led to the replacement of the crystal, which flooded the body of the Doctor and the Great One with radiation. This caused the death of the Great One and the regeneration of the Doctor into his fourth incarnation. (TV: Planet of the Spiders)
Iris Wildthyme encountered the Eight Legs on the planet with her companion Timmy. However, Wildthyme had a habit of claiming the Doctor's adventures happened to her so the truth of this is unclear. (PROSE: The Scarlet Empress)
Feeling slightly depressed, the Seventh Doctor revisited Metebelis III to try and revive his spirits, hoping the beauty of the planet would give him something to appreciate and the danger would revive his interest in life. This goal was almost too successful when he was captured and nearly eaten by one of the surviving Eight Legs, but he was saved by his next incarnation as he paid a brief visit to his previous self. The Eighth Doctor used the Tissue Compression Eliminator he had recently stolen from the Master to kill the Eight Legs before it could eat them both. (PROSE: The Eight Doctors)
During an encounter with Faction Paradox, the Doctor's history was changed so the Third Doctor regenerated on the planet Dust instead. The Eighth Doctor eventually learned the truth about the Faction's interference thanks to the interference of the TARDIS, allowing him to restore his original history and ensure the circumstances of his third incarnation's death occurred on Metebelis III. (PROSE: Interference - Book One, Interference - Book Two, The Ancestor Cell)
The Eleventh Doctor used a Metebelis crystal to boost the natural psychic abilities of Emma Grayling, in order to create and sustain a wormhole to a pocket universe and to rescue Hila Tacorien. (TV: Hide)