A Graske has stolen a key to the TARDIS. He's had it copied and scattered the copied keys around the universe. The Eleventh Doctor's mission is it to track down all keys before his enemies use them to steal the TARDIS.
The Sontarans have crash-landed in Victorian England and are building a massive cloning machine in the sewers. The Doctor has to collect ten cogs to stop them completing their machine and five missing TARDIS keys to stop them and reach the TARDIS.
Because of the ubiquity of this race, no attempt will be made here to list every appearance in every medium. Instead, we focus on game appearances here, and invite you to explore a complete list of appearances elsewhere.