Luke Rattigan was a gifted and ambitious teenager who developed the ATMOS car technology in cooperation with the invading Sontaran race. He was the founder of the Rattigan Academy, a school for geniuses. However, he developed an irrational hatred for people he deemed less intelligent than him and willingly committed global treason in order to reconstruct humanity in his own perfect vision.
Luke attended primary school between 1990 and 1992. (TV: The Poison Sky) He was already developing professional-level software by the age of 10, and at the age of 11 was offered a job by International Electromatics; he turned it down, instead choosing to work on a project of his own. (PROSE: The Secret Lives of Monsters) At twelve years old, he 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 Rutans.
In September 2005, Rattigan was given an IQ rating of 174 by Mensa experts, pioneering a new advanced IQ rating system. He went on to establish the Rattigan Academy in 2006, where he would work and reside. (TV: The Poison Sky)
The Tenth Doctor arrived when the then-eighteen-year-old Rattigan was running Rattigan Academy, teaching similarly gifted teenagers handpicked by Rattigan. (TV: The Sontaran Stratagem) 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. (TV: The Poison Sky)
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. (TV: The Sontaran Stratagem) 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, killing 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 said, "Something clever." (TV: The Poison Sky)
Even though Luke was brilliant (having an IQ rating of 174), 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 and did not understand why they refused to join his vision to save the "ordinary" people they loved. He was very ambitious and callous towards people he felt did not meet his standard of intelligence, viewing the simultaneous deaths of 52 people via his own ATMOS system as amazing rather than horrible. He also viewed his home planet of Earth as a disappointment, stating that it was never big enough for him. 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. (TV: The Sontaran Stratagem / The Poison Sky)
Behind the scenes
As revealed by writer Helen Raynor in the commentary for The Sontaran Stratagem, Luke was named after Luke Marlow, a young Doctor Who fan she knew, and in fact that was his original surname, as well. Russell T Davies, however, preferred the name Rattigan.