Klein's Story by John Ainsworth and Lee Mansfield
Elizabeth Klein is an anomaly. A renegade from an alternate future in which the Nazis won World War II. In an attempt to get to know his latest companion, the Doctor invites Klein to tell him how exactly she came to be in possession of his TARDIS and of the events that led to her trip into the past to Colditz Castle.
This story is presented as an exclusive free episode as part of the 2016 Paul McGann weekend, but can also be found as part of Doctor Who - Survival of the Fittest. It will only be available to download free until the end of May 2016, so grab it while you can!