After the Doctor's regeneration, the TARDIS crash lands back on Earth.  Rose gets the Doctor to Jackie's apartment where he is in some sort of coma.  The vortex energy draws aliens to the Earth which the Doctor wakes long enough to fight off.  These alien's appearance also bring along the Sycorax, a race that is bent on conquest of the Earth for slave labor now that they know it is inhabited.  The Doctor thwarts the invasion with a little "help" from the Prime Minister and Torchwood.