Talk about it here.
"Mr Song", also known as "the Doctor", was a renegade member of the ancient Kamishi species, also known as the First Race.
He tried to stop his race's spore ships and allied himself with the group calling themselves the "Rulers of the Universe". With their help, he abducted River Song and tried to convince her that he was in fact an incarnation of the Doctor. He encouraged her to investigate the spore ships. (AUDIO: Signs)