The Cylons request a meeting to sign a peace treaty in their 1,000 year war. Baltar is their human delegate.
A pilot, Starbuck, pretends to be sick so that his friend, Zack, can go on patrol with Zack's brother, Apollo. The two find a Cylon fleet coming to attack the negotiation meeting. They are chased by the Cylons and Zack's ship is disabled. With the Cylon's jamming their transmissions, Apollo has to leave him in order to go warn the human fleet.
Apollo and Zack are the sons of Commander Adama, who, once he loses contact with the two, urges the President to let him send more ships out to find out what happened, but Baltar convinces the President that he doesn't want to disrupt the peace negotiation. Baltar then disappears as the Cylons attack and shoot down Zack's ship as they move against the main fleet.
Apollo makes it back just as Adama orders the other Vipers (fighter ships) to launch in defense.
The Cylons destroy the Atlantia, the President's ship, killing him, and Adama realizes that the Cylons are also attacking their 12 home worlds simultaneously.The worlds and all other ships are destroyed. Galactica is the only one left.
Adama and the Galactica try to save the planet Caprica, but they're too late.
Apollo flies Adama down to the surface, where he recovers some photos of his wife from his destroyed home. His wife died in the attack.
Crowds of survivors come to them for help. One of them is a reporter named Serina. Adama tells them, and all survivors from all of the 12 worlds, to join the Galactica as it leaves the galaxy in search of a new home, the fabled 13th world, Earth. The fleet is now a total of 220 ships, only the Galactica being a warship.
We then see that the Cylons are being commanded by Baltar.
We also see that Starbuck and Athena, Adama's daughter, are in somewhat of a love relationship, but now Athena doesn't want to be with him because she's afraid she'll lose him like she lost her brother, Zack.
After two days, people are out of food and water.