Battlestar Galactica (2004)

With only each other to rely upon, and Cylon spies in their midst, trust becomes the survivors' most valuable asset...

With the human race reduced to fewer than 50,000 people, a ragtag fleet of ships searches for the mythical planet Earth, protected only by the Galactica. Newly sworn-in President Laura Roslin and Commander William Adama, joined by an uneasy alliance, lead the fleet of shocked and grieving survivors deep into uncharted space with their tormentors, the Cylons, in pursuit.

Because some Cylons now appear to be human, no one can be sure whom to trust. One of the Galactica's pilots - Lt. Karl Agathon, call sign "Helo" - experiences a Cylon deception more intimately than most. Stranded on the devastated and Cylon-occupied planet Caprica, he falls in love with a woman he thinks is his flight partner, Sharon Valerii (call sign "Boomer"). In fact she is a Cylon, one of many copies of the "Number Eight" Sharon model. Her mission: to seduce Helo and bear a child so that the Cylons can study hybrid reproduction. This plan backfires, however, when Sharon and Helo fall in love, and her devotion to him eclipses her loyalty to the Cylons.

Unknown to Helo, "Boomer," the Sharon he had served with, is still on the Galactica - unaware of her own Cylon nature, and romantically involved with the ship's senior noncommissioned officer, Chief Galen Tyrol.

Meanwhile, vivid and disturbing visions begin to haunt President Roslin. As a result, she becomes a devout believer in the mythological planet Earth, which she feels is a real place, a safe haven to which she is destined to lead humanity according to the prophecies of the Colonies' gods. Accordingly, she orders pilot Kara Thrace to hijack a captured Cylon ship, sneak back to the ruined capital city of Caprica, and retrieve the ancient Arrow of Apollo.

At a critical moment, after the Sharon aboard the Galactica returns from a critical mission to destroy a Cylon base ship, her Cylon programming abruptly activates: in one shattering instant, on the command deck of the Galactica, she shoots Adama.