Luke Rattigan is the tertiary antagonist in the Doctor Who episodes "The Sontaran Stratagem" and "The Poison Sky". He is a child prodigy who created the ATMOS car technology, and later conspires with the Sontarans.
At twelve years old, Luke invented the Fountain Six search engine, which made him a millionaire almost overnight. In his later teens, feeling misanthropic and unappreciated by society, he worked with the Sontarans to develop the "Atmos" car device, which effectively eliminated all CO2 emissions of a car. Unbeknownst to its unwitting users, ATMOS would also poison the atmosphere, allowing the Sontarans to build a clone army to continue their war against the Rutan Host.
The Tenth Doctor arrived when the then-eighteen-year-old Rattigan was running his own Rattigan Academy, teaching similarly gifted teenagers handpicked by Rattigan. Unbeknownst to any of them but Luke, the Sontarans had promised to give them the nonexistent planet "Castor 36" (renamed "Earth-Point-Two" by Rattigan) to build a new world after the poisoning of Earth.
The Sontarans activated "Atmos" which had been installed into 50% of the world's vehicles. The Atmos systems emitted a gas which would kill all humans for their cloning process. Clouds soon wrapped the whole planet. Overcome by the thrill of war, Luke teleported from the Sontaran ship, where he'd been watching the gas enveloping the planet, back to his academy. He tried to convince his pupils to come with him to "Castor 36" to found their own civilisation (for which they had been developing super-advanced technology in their labs). Horrified by the disaster and taking Rattigan to be mad, they abandoned him and fled.
A dejected Luke returned to the Sontaran mothership and reported his failure. The Sontarans revealed they had had no plans to keep the students alive anyway; they had merely needed Luke and his pupils for the creation and installation of the Atmos systems. After that, their only plans for the students were to use them as target practice.
Devastated, Luke teleported back to Earth. He was met again by the Doctor, who told him to "do something clever with his life". The Doctor teleported to the Sontaran ship with a reversed atmospheric converter. He intended to blow up the Sontarans — and himself — if they refused to leave Earth alone. Taking the Doctor's advice, Luke reprogrammed the teleporter, switched places with the Doctor, and blew up the Sontaran ship just in time, sacrificing himself. When Martha and Donna (who were present on Earth with the Doctor) asked him what he was doing when he reprogrammed the teleporter, he replied, "Something clever." before switching places with the Doctor and shouting "Sontarans? HA!" as a mockery of the Sontaran's victory chant.
Even though Luke was brilliant (actually receiving an IQ rating of 176), he acted neurotically, in outbursts of unreasoning emotion. Luke lacked social skills. He believed all his students had been rejected from society for being intelligent. He was very ambitious and callous towards people he felt did not meet his standard of intelligence. He found the Doctor particularly annoying because of his cheeky attitude, his obviously superior intelligence and his use of tautology. Luke joined forces with the Sontarans because they seemed to appreciate his genius, leading him to commit global treason. When the Sontarans turned against him, Luke redeemed himself by sacrificing his life to destroy them.