The Host

Small parasitic aliens called "Souls", who travel to planets inserting themselves into a host body of that planet's dominant species while suppressing the host's consciousness, have taken over the human race. Deeming humans too violent to deserve the planet, they have now almost successfully conquered Earth. The original owner's consciousness is erased, but the Souls can access the host's memories, and occupied hosts are identifiable by silver rings in their eyes.

A human on the run, Melanie Stryder, is captured and infused with a Soul called "Wanderer", whom a "Seeker" soul has asked to access Melanie's memories and learn the location of a pocket of unassimilated humans. Melanie's consciousness, however, has not been completely eliminated; she and Wanderer carry out an internal conversation and debate with each other, eventually becoming friends.