In a parallel universe, Flint was a soldier. He came to search Eric Vollmer's research. He praised Lee's work, but he was mutated by the genetic super-soldier experiments Lee had been performing.
For 27 years, Flint remained at the base, mutated and wanting to die. He was left for dead by the Doctor, but he knew the Doctor would return for his TARDIS and so waited for the Doctor to come. Full of hatred for the Doctor, Flint force-fed him the TARDIS key and snapped his neck, causing him to regenerate. He blew up the base. (AUDIO: Full Fathom Five)