The Extron were a parasitic species.
An Extron imprisoned by the Judoon on Inkarsera escaped and arrived on Earth in 2007, hiding in Violet Hopley's bedroom. The Tenth Doctor tracked it there, but the Extron kidnapped the girl before he could stop it. Violet was brought to Onla-toch, whose civilisation the Extron had destroyed years before.
The Doctor managed to figure out where Violet had been taken after researching the records on Inkarsera. He got Violet to put biometric handcuffs on the Extron before it could finish full gene transmutation using her energy pattern. The handcuffs transported the Extron back to Inkarsera, and the Doctor returned Violet home. (COMIC: The Monster Upstairs)