Razor takes place during the first 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 Helena 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.
Lee Adama's first priority as Pegasus commander is to restore trust and integrity to the troubled command he has taken while preserving some continuity in the leadership that had evolved under Rear Admiral Cain. He picks Kendra Shaw as his executive officer, promoting her to major, despite the stark contrast between Cain's enthusiastic praise and subsequent Pegasus officers' complaints about Kendra's performance. First boarding Pegasus less than an hour before the Cylon attack on the colonies, Shaw follows Rear Admiral Cain's order to make a "blind jump" that saves the ship from destruction at Scorpion Fleet Shipyards.
Kendra Shaw, as did Gaius Baltar on the Galactica, discovers that the Cylons were about to infect the Pegasus with a computer software virus which, if made operational, would have disarmed and thereby destroyed the Pegasus.
After Pegasus receives reconnaissance information that all the colonies have been destroyed, Cain delivers a rallying speech inciting the crew to war for the purpose of revenge. The crew responds by chanting "So say we all!" throughout the ship; the still-unexposed Number Six Cylon agent Gina Inviere awkwardly joins in.
Lee Adama's first mission as commander of Pegasus is a search and rescue mission to retrieve a lost science team. Antiquated Cylon Raiders (the ships and Cylon pilots who seem strongly reminiscent of those from the original series) from the first war appear and attack Pegasus. Simmering bitterness between Kara and Shaw is exacerbated when Shaw endangers Kara and her wingman in order to defend the battlestar. Both pilots land safely, though Kara has to destroy a Raider that follows her into Pegasus.
Later, during a heated exchange, Shaw reminds Kara that defying authority is a risky proposition aboard Pegasus, recalling how Cain shot her executive officer, Colonel Jurgen Belzen, for refusing an order to attack a Cylon staging area filled with an unexpectedly strong force of Raiders. It is during this mission that Gina Inviere helps Cylon centurions board the Pegasus and Shaw discovers Gina's true identity. It is hinted that the sadistic torture inflicted on Gina by Cain's orders had been influenced by the burgeoning romantic relationship between Gina and Cain. When Shaw is promoted to captain by Admiral Cain, she reminds her troubled protégé that the Cylon tricked them all.
Viewing the wreckage of the raider, Sharon Valerii notes that Cylon lore tells of a group of original centurions tasked to guard the first hybrid constructed by the Cylons as a result of their human experiments. Admiral Adama then recalls his rookie mission as a Viper pilot, when he encountered that hybrid and saw what the Cylons did to their human captives in the course of their research. Admiral Adama concludes that the Cylons must still be pursuing the experiments, with the captured science team as new test subjects.
Lee Adama approves a plan to use Pegasus to find the basestar and lure the Cylon Raiders away from it, then launch a rescue team — including Kara Thrace and Kendra Shaw — outfitted with a nuclear warhead, in a Raptor. Admiral William Adama transfers his flag to Pegasus to counsel his son on his first command. The diversion succeeds; the rescue team sacrifices its Raptor and boards the basestar.
As the rescue team searches the basestar, the Cylon hybrid can be heard speaking in more coherent phrases than its more-modern counterparts. It repeatedly utters the words "All this has happened before, and will happen again." The hybrid also makes a far more elaborate pronouncement:
At last, they have come for me. I feel their lives, their destinies spilling out before me. The denial of the one true path, played out on a world not their own, will end soon enough. Soon there will be four, glorious in awakening, struggling with the knowledge of their true selves. The pain of revelation bringing new clarity. And in the midst of confusion, he will find her. Enemies brought together by impossible longing. Enemies now joined as one. The way forward at once unthinkable, yet inevitable. And the fifth, still in shadow, will claw toward the light, hungering for redemption that will only come in the howl of terrible suffering. I can see them all. The seven, now six, self-described machines who believe themselves without sin. But in time, it is sin that will consume them. They will know enmity, bitterness, the wrenching agony of one splintering into many. And then, they will join the promised land, gathered on the wings of an angel. Not an end, but a beginning.
The science team is found and partly rescued, but the rescue team comes under fire from Cylon centurions. While the team manages to evacuate the basestar in a second Raptor, the nuclear warhead is damaged and has to be detonated manually. Wounded, Shaw remains behind to detonate the warhead and comes face to face with the Cylon hybrid.
The hybrid offers absolution to Kendra, who is haunted by the fact that it was she who fired the first shot in the massacre aboard the civilian ship called Scylla as part of an operation to conscript civilians for the Pegasus. The hybrid then tells her that "Kara Thrace will lead the human race to its end. She is the herald of the apocalypse, the harbinger of death. They must not follow her." Shaw tries to relay this information to Lee Adama, but is stopped when the Cylons jam communications. Her final act is to detonate the nuclear warhead, destroying the basestar.
On Galactica, Admiral Adama reflects on Cain's legacy and proposes a posthumous commendation for Kendra Shaw, over his son's protests that she and other members of Admiral Cain's crew "butchered innocent civilians". They close their arguments, however, noting that history's first draft will be written in their logs, and Lee agrees to consider the commendation.
The following are listed in order of their prominence on the DVD front cover:
Michelle Forbes — Helena Cain
Stephanie Jacobsen — Kendra Shaw
Mary McDonnell — Laura Roslin
Edward James Olmos — William Adama
Katee Sackhoff — Kara "Starbuck" Thrace
Jamie Bamber — Lee "Apollo" Adama
James Callis — Gaius Baltar
Tricia Helfer — Number Six / Gina Inviere
Grace Park — Number Eight / Sharon Valerii
Michael Hogan — Saul Tigh
Fulvio Cecere — Alastair Thorne
Nico Cortez — Young William Adama, callsign "Husker"
Steve Bacic — Jurgen Belzen
Graham Beckel — Jack Fisk
Campbell Lane — Hybrid