Stephen, a young boy with a cold, is wandering the streets of London in April 2010 when he sees an alien spaceship fall out of the sky. As people look inside the wrecked craft, giant Space Leeches fly out and attach themselves to the investigators' heads. The leeches swarm, hundreds of them. Remarkably, Stephen escapes as everyone around him is taken over. He runs to the top of Primrose Hill and decides he needs help. Then the TARDIS materialises around him...