The show starts with Watson on the Jubilee line, showing off his new moustache.
He then zooms along the District line.
And pops out by Euston Square station, which is on the Hammersmith & City and Metropolitan lines.
Once Watson is reunited with Sherlock, the pair look at a strange video. It shows a suspicious man attempting to travel one stop on the District line from Westminster by using a Jubilee line train.
But the man has vanished by the time his Jubilee line train reaches St James's Park. Which isn't that surprising, since St James's Park is only on the District line
Confused by the fact that London Underground trains are pulling into stations on completely different lines, Sherlock and Watson rewatch the footage.
And Sherlock concludes that the "five minute" journey between Westminster and St James's Park must have somehow been made to last ten minutes.
The fearless investigators decide to get involved and avert a disaster. So they pop into Westminster station on the Jubilee and District lines.
Then there's a sign for the Northern Line in the background.
And then they're in the abandoned Jubilee line station at Charing Cross, where Sherlock and Watson climb down a ventilation shaft.
Which they use to access a District line train...
...by running along a disused section of the Picadilly line...
...from the former Aldwych station.
Ultimately, the shape-shifting London Underground network is an even bigger mystery than working out how Sherlock survived his fall.
