Swimming through a huge giant organism towards a heart, the Doctor and Rose, suited up and equipped with giant surgical appliances, encounter an alien organism feeding off the chamber wall. Once the organism is disconnected (i.e. cut away) from the wall, they are attacked by the white blood cells of the immune system. These start to attach themselves to the Doctor and Rose, swamping them. The only way to escape is to enter the organism and be eaten by it. As the organism begins to absorb them, it too is swamped by the attacking white blood cells. Overcome, the organism explodes.