These days we’re always moaning when freezing weather brings everything to a grinding halt, but back in 1814, the weather was so cold in London, the River Thames actually froze over. The locals didn’t whinge, however – in fact they celebrated with a ‘frost fair’ on the ice with dancers, acrobats and elephant shows. But as history records show, this was the last year it ever happened and in this week’s Doctor Who, viewers find out why…

There’s something very odd and sinister going on below the fair. People start disappearing, sucked under the ice by weird glowing fish, then fed to a gigantic serpent that lies on the river bed.

Luckily, the Doctor and Bill have travelled back to the Regency era, so they can look into the mystery. The TARDIS wardrobe has helpfully provided them with fashions of the Georgian era – a top hat for the Time Lord and Bill gets a long velvet coat and plumed headwear. And to explore the murky depths of the Thames they try out those old-style diving suits – definitely pre-Jacques Cousteau apparatus!

Teaming up with a group of street urchins, the pair eventually cross paths with a nasty aristocrat, Lord Sutcliffe (Nicholas Burns), who gets a well-deserved wallop from the Time Lord. The enormous serpent, however, if is a different beast altogether and it will take more than a well-aimed punch to deal with it…