The film unfolds in a distant star system set in the seventh millennium of time, where the humans from the Twelve Colonies—each named after astrological signs like Gemini and Caprica—are locked in a fierce battle against the Cylons, a relentless robotic race intent on wiping out humanity. A surprising twist occurs when the Cylons, through the duplicitous Count Baltar (John Colicos), seek an end to the hostilities. The Council of the Twelve, representing the different colonies, along with military leaders, eagerly accept the Cylon offer, delighted to finally put an end to the protracted conflict.
However, this gesture of peace turns out to be a deceitful ploy. The Cylons harbor no intentions of reconciliation, and Baltar has betrayed mankind, ensuring the survival of his own colony at the cost of all others. Commander Adama (Lorne Greene) of the Battlestar Galactica remains skeptical, suspecting a trap, and dispatches a reconnaissance mission led by his son, Captain Apollo (Richard Hatch), along with Lieutenant Starbuck (Dirk Benedict). When Adama's youngest son, Zac (Rick Springfield), persuades Starbuck to let him take his place on the mission, disaster strikes as Zac's fighter is attacked and destroyed by the Cylon fleet after they reveal an enormous armada hidden near the moon Cimtar.
In light of the betrayal, Adama faces a dire situation as President Adar (Lew Ayres) is manipulated by Baltar into delaying the fleet’s response, leading to the devastating Cylon assault on the Colonial fleet. As the Galactica manages to prepare its Vipers for battle, the other battlestars fall victim to the surprise attack. Amidst fierce conflict, the Galactica emerges as the sole remaining bastion, escaping to the protection of Adama’s leadership. Following the destruction of their home colonies, Adama gathers the survivors and commands all intact civilian vessels to join them on a perilous quest for the legendary thirteenth colony, known as Earth—a mysterious destination known only to the ancient rulers of Kobal.
The perilous journey leads the ragtag fleet through the hazardous Nova of Madigon, skirting danger in their attempt to evade Cylon forces. After a brief, uneasy respite on the resort world Carillon, where food and fuel become scarce due to contamination from the Cylon attack, the humans quickly realize the insectoid inhabitants may not have pure intentions. The Cylon Imperious Leader has put a hit out on Baltar to ensure no humans survive, yet the cunning Count remains alive under coercion due to his strategic value.
As the Council of the Twelve, manipulated by Sire-Uri (Ray Milland), tries to undermine Adama’s leadership by proposing a disarmament plan to feign peaceful intentions towards the Cylons, the Commander remains vigilant. He suspects a trap lies in wait during their ceremonial banquet, suspecting it’s a perfect opportunity for a Cylon ambush. Meanwhile, Apollo and Starbuck uncover the frightening truth—Carillon's residents are luring humans to serve as food for their larvae. In a desperate bid for survival, the humans engage in a fierce confrontation with Cylon centurions, resulting in a catastrophic tylium mine fire that threatens to destroy the planet.
As the Cylons initiate their attack on the Galactica while believing the fleet's pilots are attending the banquet, Adama cleverly uses this distraction to turn the tide in their favor. Apollo and Starbuck, defying orders, discover a hidden Cylon basestar and use ruse to divert its attention, leading it to fall into the planet's raging inferno. Despite their hard-fought victory, the humans keenly realize the Cylons will still pursue them, as they set their sights on Earth, their last beacon of hope.
