Razor takes place during the first thirty days of Lee Adama's Pegasus command during the period originally covered in the second half of season 2, after the episode "The Captain's Hand" but before "Lay Down Your Burdens", and uses flashbacks to show Rear Admiral Cain's orders during and shortly after the Cylon attack, as well as events occurring on the final day of the first Cylon War. Cain's actions in the wake of the holocaust are connected to the present by Kendra Shaw, self-described as Cain's legacy.

Events of the final day of the first Cylon war are centered on then-Lieutenant William Adama, a Viper pilot, and Helena Cain, a youngster at the time. Flashbacks of Adama discovering a lab housing the early Cylon experiments on humans as well as the first Cylon hybrid and Helena Cain abandoning her little sister during a Cylon invasion of Tauron both end as the first Cylon war ends immediately after the life-changing events, leaving each character hanging somewhat (Adama is unable to free the surviving prisoners and Cain cannot find her sister). These are shown to be driving forces for the actions of both, as the show continues. Cain goes on to teach Shaw that you have to become a 'razor' to survive during war while referring to her pocket knife, which later becomes Kendra's and then is passed on to Kara Thrace at the end of the show. Admiral Adama teaches his son Lee a different lesson: that they cannot become butchers.