The TARDIS brings the Doctor and Martha to Balmoral in 1902. Here they meet Captain Harry Carruthers - friend of the new king, Edward VII. Together they head for the castle to see the king - only to find that Balmoral Castle is gone, leaving just a hole in the ground. The Doctor realises it is the work of the Judoon - a race of ruthless intergalactic mercenary space police.
While Martha and Carruthers seek answers in London, the Doctor finds himself in what should be the most deserted place on Earth - and he is not alone.
With help from Arthur Conan Doyle, the Doctor and his friends discover a plot to take over the world. With time running out, who will fall victim to the revenge of the Judoon?