A man wanders out of the desert not knowing who he is. His brother finds him, and helps to pull his memory back of the life he led before he walked out on his family and disappeared four years earlier.

When the initial Cylon attack against the Twelve Colonies fails to achieve complete extermination of human life as planned, twin Number Ones (Cavils) embedded on Galactica and Caprica must improvise to destroy the human survivors.