R is for... Rattigan

Luke Rattigan was a teenage scientific genius, best known for inventing ATMOS, a system which effectively removed carbon dioxide from vehicle emissions. 

He had begun by inventing new computer software aged only 10, and by 12 he had created the successful Fountain Six search engine.

He used his wealth to set up a private academy in North London, which accepted only the most gifted of students. Much of the work they did was around space exploration, including terraforming technology.

He had secretly entered into a partnership with the Sontarans, who sought to exploit Earth's pollution to turn it into a clone-breeding planet as part of their long-running war against the Rutans. ATMOS was really designed to pump out the gases necessary to change the atmosphere of the planet. 

It also had a built-in satellite navigation system, which could be overridden to eliminate anyone who tried to investigate what Rattigan was doing, such as journalist Jo Nakashima.

General Staal of the Tenth Sontaran Battle Fleet had a flagship in hidden orbit above London, and had set up a teleport unit at Rattigan's academy, as well as another at the ATMOS factory. 

The latter was investigated by UNIT after a number of simultaneous deaths globally, all involving the device.

Rattigan had been promised a new life with his students on the planet Castor 36. 

When ATMOS was activated across the planet, he found that his students were unwilling to abandon their families and follow him. When he reported this to Staal, he was shocked to learn that the Sontarans had never intended to honour their promise.

The Doctor elected to use a terraforming device to destroy the Sontaran flagship, even though it would mean his own death. Seeking revenge, his dreams of a new society shattered, Rattigan swapped places with him at the last moment and activated the device.

Played by Ryan Sampson. Appearances: The Sontaran Stratagem / The Poison Sky (2008)

• Sampson began his acting career at the Crucible Theatre in Sheffield, but is best known for long running TV comedy roles, including Brassic and Plebs.
• He was a fictionalised Charles Dickens in The Frankenstein Chronicles, and played Dudley Moore in an episode of The Crown.
• More recently he has been seen as Mr Collins in The Other Bennet Sister.