He set up an academy to teach other people about his genius, and called it 'Rattigan Academy'. Creative.
He also allied himself with the Sontarans, and used their technology to create a device called ATMOS, which apparently stands for the 'Atmospheric Emission System'. He is very touchy about people calling it the ATMOS System, because he is offended by the fact that, technically, people are thne saying the Atmospheric Emission System System. Hmm.The ATMOS System (trololol) is designed to reduce all carbon emissions from cars to 0. However, it also has a secondary function of releasing a seemingly endless cloud of Sontaran Clone Feed Gas, which is toxic to Humans. The Sontarans wanted to use the ATMOS System to pollute the entire of planet Earth with their gas, kill all the population and then transform the planet into a Cloning World.
Not like Luke knew any of this. He thought they just wanted to kill all the Humans because they are so stupid, and he was promised by the Sontarans that he would be given a new planet to populate with his students. When he told his students of this, they simply left. Devastated, he returned to the Sontarans and told them of his failure. It was then that the Sontaran General Staal revealed that his students were simply going to be used as 'Target Practice' (Yes, because the Sontarans apparently don't have Target Practice Systems on board their ships. That explains a lot.)Rattigan returned to Earth via teleport and was confronted by the Doctor, who had visited the Academy several times before with UNIT to inquire about ATMOS. The Doctor used all the tech gear lying around to construct an atmospheric converter which cleansed the sky of the Sontaran Gas. When Staal realised this he ordered his ship to destroy the Earth, but the Doctor teleported into the Sontaran ship after telling Luke to do 'something clever with his life'. The Doctor threatened the Sontarans with the converter, telling them that he had it set to ignite all the Sontaran fart gas in the ship. The Sontarans merely laughed at him, saying that they did not fear death and they were happy that, in killing the Sontarans, the Doctor would kill himself.
However, Luke was finally doing something clever with his life. He re-wrote the teleporters software to switch the Doctor and him, so that the Doctor was saved and he was dropped onto the Sontaran ship. As Stall looked on in horror, he simply said 'HA!' before detonating the ship, killing all the Sontarans aboard, and himself, and saving the planet.