DavidHeath9
Space and time constantly splitting into what is referred to as the multiverse opens an endless stream of possibilities, for example the Doctor not regenerating the way he was supposed to.
The Alternate Doctor is a result of the Twelfth Doctor passing by a time storm mid regeneration, frying the time circuits in the TARDIS and throwing off his regeneration. Now instead of regenerating into 13 like he was supposed to, now he's splintered off into an alternate timeline with a face he never was supposed to have. Somehow he can tell something about the form he's taken is wrong, and he has to find out how, if at all, he can fix it.