Complex sentence: Wandering back to his TARDIS at the South Pole, the First Doctor, refusing to regenerate and leaving his companions behind, encounters the Twelfth Doctor outside his own TARDIS, in a similar state of mind after his previous ordeal.
Split into simpler sentences: Wandering back to his TARDIS at the South Pole, the First Doctor, refuses to regenerate, and leaves his companions behind. He encounters the Twelfth Doctor outside his own TARDIS in a similar state of mind after his previous ordeal.