While not a multi-part story in the traditional sense, New Earth and Gridlock - which were broadcast a year apart from each other - are strongly linked. Not only does Gridlock (a Series 3 episode) bring the TARDIS back to New New York, but it also continues the stories of supporting characters like Hame and the Face of Boe, who both met the Tenth Doctor during New Earth in Series 2.
As a result, Gridlock can be described as a sequel to New Earth (which, in turn, can be described as a sequel to The End of the World, the Face of Boe's first appearance), making this one of the only "multi-part stories" to air across different seasons.
And that's exactly why these two episodes were chosen for a recent watch-along event, which was attended by the man who wrote them both - Russell T Davies.
Over on his Twitter account, he discussed the scripting and production of both adventures at great length, dropping a host of cool details along the way. From CGI shortcuts to Star Wars influences, here are some of the best things he revealed!