The as-yet-untitled story, which is expected to air on Christmas Day, will guest star comedians Matt Lucas and Greg Davies and will also feature the return of River Song (Alex Kingston), who was last seen opposite Matt Smith's Eleventh Doctor in 2013's The Name of the Doctor.
On his decision to bring back River to the show after a two-year absence, Moffat says:
“I really fancied it. I hadn’t written River for a couple of years and I’d missed her. I thought, given that I’m knackered at the end of a series, what would be a treat for me would be getting Alex Kingston back onto Doctor Who.”
He also reveals that the decision was partly prompted by the belief that it could be his last Doctor Who episode as showrunner:
“We’ve just lost Clara, so I didn’t want to go straight into a new companion. I’ll be honest, I brought River Song back in because I thought there was a possibility I’d never write it again so that’d be my goodbye.”
Having previously described River and Peter Capaldi's Twelfth Doctor as 'lovely together', Moffat also elaborates on the new dynamic between the two:
"It’s strange because they’re now, in our human terms, the perfect couple. They’re both sexy older people. It’s always been slightly strange before with Matt Smith. Now, yeah, they could actually be married. It works."
And on the lighter tone of the episode itself, which sees the Doctor 'hurled into a fast and frantic chase across the galaxy', Moffat describes it as a 'great romp for Christmas Day':
"If you're a long term fan of the show you might be interested because there's been a lot of talk about the Doctor and River's glory years but we've never seen them. Here at last we see them as Mr and Mrs Who at the height of their powers. It's great fun."