Doctor Who: Aliens of London

A space-ship crash-lands in the Thames, but not before taking off one of the faces of Big Ben. The inhabitant is rescued, still alive but as it tries to escape, the army kill it. It resembles a pig, and the Doctor deduces that this is exactly what it is, augmented to appear alien. Meanwhile, the prime minister has mysteriously disappeared and as his cabinet meet to discuss the situation it becomes clear that the cabinet have been taken over by a race called the Slitheen, huge bestial creatures made to inhabit smaller human forms by a compression field. Convincing the peoples of Earth that the pig-like aliens have weapons of mass destruction, they guide the planet to the brink of war.