When the sky rips open somewhere over Dover, two objects hurtle out of the Vortex and crash-land in the sea. One is the TARDIS, out of control and freefalling - but the other, a mysterious crystalline sphere, is far more sinister. The Doctor and Donna are rescued and taken to a secret command centre in the Dover cliffs. It's May 1940, and Vice-Admiral Ramsey is about to finalise one of the most daring plans of the Second World War: Operation Dynamo.