Stoyn

Quadrigger Stoyn was a Time Lord engineer who was decommissioning the Doctor's TARDIS when the First Doctor and Susan Foreman first stole the machine, and travelled with them to Earth's moon. He was angry as he was in the engine room when the Doctor made his first journey and was scared of being on a different planet. When a stasis field was released by the TARDIS, he told the Doctor that he had ruined the Archaeons' experiment, but wanted to leave the Doctor on the Moon and take Susan travelling with him. (AUDIO: The Beginning)

Stoyn stayed on the Moon for some time but found that the noise of too many people disrupted his focus. Stoyn left the Moon and found a sentient planet far away from civilisation. The planet created the circumstances from which Stoyn could track the Doctor's travels through space, and he created a wormhole drawing the TARDIS to him. When the TARDIS arrived, Stoyn encountered the Second Doctor, Jamie McCrimmon and Zoe Heriot. He wanted to drain the TARDIS of its energy. He later asked Zoe to help him build a machine. He was trying to make a hypercube. (AUDIO: The Dying Light)

Romana I and the Fourth Doctor visited Rome during their search for the Key to Time and tracked the sixth segment there. Stoyn presented a play in the streets of Rome called Luna Romana. He found the TARDIS in the temple of Luna and told Romana that he had been to the moon once. He thought that Romana was his only hope.

The Randomiser later took Romana's second incarnation to a recreation of ancient Rome on Earth's moon, where she was captured by Stoyn, who had been trapped there for 2000 years. Stoyn captured the Fourth Doctor as well, and showed his prisoners pictures of their travels which he'd gathered via components stolen from the Doctor's TARDIS; during his exile on the Moon, he'd only been able to watch the Doctor's travels. After Romana stabilised a time field, Stoyn accidentally used it to travel back to ancient Rome where he met the first Romana, experiencing her earlier adventure from his perspective.

Stoyn was furious upon learning from Romana that his job had since been replaced by a single button in TARDISes post-Type-54, and there were only a few temporal mechanics left. He tricked Romana into allowing him into the TARDIS, using the fifth segment of the Key to Time as bait. Stoyn dismantled K9 Mark II, having found the robot annoying. He declared the TARDIS property of the capitol and intended to take it back to Gallifrey and ensure his legacy. Romana said that Stoyn was remembered in the Academy, albeit as an expression for a mistake in engine repair. Stoyn, believing that the Doctor and Romana were betraying the legacy of Gallifrey itself, wanted to rewrite time so that the Doctor wouldn't steal the TARDIS in the first place.

Stoyn was killed when an accident caused an explosion in the controls at his laboratory on the Moon, dispersing him as the temporal energy he would have used to erase the Doctor's travels was instead converted into solar energy that disintegrated him. (AUDIO: Luna Romana)

Legacy
During her Presidency of Gallifrey, another incarnation of Romana decommissioned Stoyn's Quadrigger station on Gallifrey so that the land could be used to build Battle TARDISes. She visited Stoyn's abandoned station just before it was destroyed, remembering her past encounters with him. (AUDIO: Luna Romana)

Behind the scenes

 * In a talk at the Doctor Who Panel of the Edinburgh Sci-Fi Con2018, Terry Molloy expressed an interest in Stoyn returning for further storylines, suggesting that Stoyn could be trapped inside the TARDIS itself trying to find some new way to take control of the ship and return to Gallifrey.