Maisie, who plays Arya Stark in the HBO hit show, turns up to surprise the Timelord, who sounds astonished to see her... "You???!!!" he bellows, after she greets him with, "What took you so long, old man?"
The internet is already abuzz with speculation over the identity of Maisie's character - is she the Doctor's daughter? Susan? Jenny? We'll have to wait until mid-September to find out, with a release date for the new series also released - 19 September.
In other news, Jenna Coleman is firmly back in action, good to see after she nearly left the series last year - but stayed on because, as she admitted at the time, she realised she'd miss being in the show too much.
Peter Capaldi said of his regular co-star, "Jenna continues to deliver a complex and funny performance that marks her out as one of the best companions ever. Will these be her last shows? I hope not."
He added on the debut of the new trailer that this series would see "a new hunger for adventure".
He shared: "Soaring through all of time and space, series nine sees the Doctor throw himself into life with a new hunger for adventure.
"The Cosmos is there for the taking, thrilling, epic and enticing, and his to play in.
"But he's almost reckless in his abandon. It's almost like he's running from something, something that if it ever catches him will turn his life upside down."
All this speculation should not divert attention from the real surprise of this first series trailer... Peter Capaldi's stunning bouffant hairstyle, a million miles from the stern crop of his first appearance as the Doctor, and more luxurious than he's appeared in screen in a long while, possibly since 'Local Hero' in 1983.
Series 9 of Doctor Who will make its debut on 19 September.