Doctor Who loves making callbacks to its own history. In the early days of NuWho, writers like Russell T Davies would reference episodes from the show's original run. This was done to provide a bit of fan service, and also to prove RTD's credentials as a massive nerd.
NuWho has now been around for so long that there are references to early modern episodes in some of the newer ones. 2005 was almost two decades ago, just to make everyone feel old.
Whether it's modern to classic, classic to classic, or modern to modern, some connections between episodes are obvious, while some take a little bit of work to figure out. Our original list on this subject (video version below) already pointed out ten of these obscure links, but there are loads more scattered throughout Doctor Who history.
Whether it's a character visiting a location that was mentioned in passing years prior, old props reappearing, or ancient plotlines getting dragged back up, these ten pairs of episodes all have something in common.
These connections can be big, small, or absolutely minute, but they're all bags of fun to notice regardless!
10. The Doctor Dances & Twice Upon A Time
The planet Villengard is located at the centre of the Doctor Who universe, and was home to massive weapons factories during the Time War. This made the planet a key battleground in the fight between the Daleks and the Time Lords.
There's a Doctor Who comic where the War Doctor goes there and turns all those weapons factories into a banana grove, because the comics are just as weird (if not weirder) than the show itself.
The Twelfth Doctor visits Villengard in his final episode, Twice Upon a Time, alongside the First Doctor, the glass avatar of Bill Potts, and Captain Archibald Hamish Lethbridge-Stewart.
Not only was this the last outing for Peter Capaldi as the Doctor, but it was also the final episode written by departing showrunner Steven Moffat. What you might not have realised is that Moffat's inclusion of Villengard made for a neat little callback to the very first Doctor Who story he wrote (The Curse of Fatal Death notwithstanding).
In Series 1's The Doctor Dances, the Ninth Doctor comments that Captain Jack's sonic blaster is from the weapon factories of Villengard in the 51st century.
Twelve real-world years later, and Moffat capped off his Doctor Who tenure by visiting Villengard with the Twelfth Doctor, creating a nice little bookend for his time on the show.