When you travel around time and space as much as the Doctor does, it's inevitable that you start to make connections between your various trips. These connections range from the big – like major sequels to classic adventures, or return appearances of the Daleks, Cybermen, Sutekh, or whoever else – to much smaller, deep-cut links. Many Doctor Who stories are connected by similar props, like the Drahvin gun from Galaxy 4 that shows up in Genesis of the Daleks.
Other times, older stories can gain a new dimension, such as when Captain Avery's appearance opposite the Eleventh Doctor retrospectively turned The Smugglers into a sequel to The Curse of the Black Spot. We've covered this sort of thing once or twice before, but there are now over 60 years of Doctor Who and it feels like the web of time's strands are becoming further and further interlinked. Hell, who could've imagined that Doctor Who's Beatles episode would contain such a direct reference to An Unearthly Child?! But that sort of connection is far too obvious.
Instead, let's take a much deeper dive into the ties that bind the Doctor's adventures. 10. Frontier In Space & Bad Wolf In Frontier in Space, the Third Doctor is accused of espionage and gets sentenced to hard time in a penal colony on the Moon.
This is in the 26th century, where the Lunar Penal Colony is reserved for political dissidents and spies. Of course, the Doctor immediately tries to escape, with assistance from Outer Space Jeremy Corbyn impersonator, .