During an experiment with a teleportation device near Ringstone, England, on 21 March 1944, a Wyrrester was transported to Earth. It was destroyed in a Nazi bombing raid, but it influenced Clearfield's mind and physiology, making him more intelligent and nearly immortal.
He spent the next 70 years preparing a new but modified teleportation device to allow the Wyrresters to return to Earth. He also experimented on insects to create hosts for the Wyrresters when they did arrive. The invasion was meant to occur on 21 March 2014 but was narrowly averted through the efforts of the Twelfth Doctor, Clara, and several others. According to the Twelfth Doctor, as Clearfield entered the teleportation machine as it was reversed, he was either scattered into atoms or sent to the Wyrrester's homeworld. (PROSE: The Crawling Terror)