Tonight's Christmas Special episode fo Doctor Who bid farewell to Peter Capaldi and Steven Moffat, but also saw the return of a former companion for a special cameo.
SPOILERS for Doctor Who "Twice Upon a Time" Follow
Pearl Mackie also gave her final performance as companion Bill Potts in this year's Christmas Special, though it was not exactly the Bill who had been traveling with the Doctor. She was actually a glass construct from the future full of Bill's memories from the moment of her death.
Just before the Doctor returned to his TARDIS to make a final decision about whether to regenerate or die, this Bill gives the Doctor a gift. She kisses the Doctor on his cheek and suddenly his memories of Clara Oswald are restored.
Jenna Coleman reprises the role for a brief cameo scene. She wishes the Doctor a Merry Christmas and warns him not to forget her again since it was so offensive the first time around.
Fans who have been following Doctor Who will remember that the Doctor had his memories of Clara removed in the season nine finale "Hell Bent." This was after the Doctor rescued Clara from the moment she was meant to die and stole a TARDIS (again) to help her escape. Clara decided that she needed the Doctor to let go and so she had herself removed from his memories before she took off in the TARDIS with Me.
Now, with his memories restored, the Doctor returned to his TARDIS where he regenerated into the Thirteenth Doctor.