Episode 1 - Scattered
With Adama out of action, Colonel Tigh takes his position as commander of the Galactica. The fleet makes an emergency jump, but due to an error by Gaeta, Galactica does not transmit the proper jump coordinates and the fleet becomes separated from Galactica. Boomer is in jail, and the cat's out of the bag - she's a Cylon. Back on Caprica Boomer takes off with Starbuck's Cylon raider.
Episode 2 - Valley of Darkness
The Galactica has succeeded in reuniting with the fleet, but it has paid a terrible price: A Cylon computer virus has penetrated its computers. Col. Tigh knows from bitter experience what the Cylons are planning. The Centurions aren't going to blow up the ship; they're going to kill the crew by venting the ship's air and then turn its guns on the rest of the fleet. Only Lee whom Tigh despises for his "disloyalty" to Commander Adama and his small squad of marines are in position to stop the Cylon boarding party. Meanwhile, on Cylon-occupied Caprica, Starbuck and Helo break into her old apartment and fire up her old pickup truck. Light-years away on Kobol, Chief Tyrol and his crew return from their risky mission to get medical supplies for Socinus, only to realize that the wounded man isn't going make it.
Episode 3 - Fragged
Adama is still in surgery. Meanwhile, the men and women on Kobol are searching for a way out of their desperate situation. They discover that the Cylons have built an anti-aircraft battery, which is designed to destroy any rescue craft. Crashdown leads the group on a raid to destroy the weapon before their rescue ship can arrive. Back on Galactica President Roslin is experiencing withdrawal symptoms after being deprived of chamalla. Colonel Tigh is facing off against the Quorum of Twelve. Fed up with their interference, he declares martial law.
Episode 4 - Resistance
Martial law is having a bad effect on the fleet as ships are refusing to resupply Galactica. The Chief is arrested as a suspected Cylon, and it's up to Gaius Baltar to prove his innocence. The President, with the assistance of Lee Adama, escapes her imprisonment aboard Galactica and takes refuge on Cloud 9 with the help of the most unlikely of allies.
Episode 5 - The Farm
A week after last week's exciting episode , Adama returns to an excited CIC and retakes command of his ship. He orders a ship to ship search to find the President, but fails to find her before she sends out a message informing the other ships in the fleet that she knows where Earth is, and that she will lead them there. Over a third of the fleet leaves Galactica's protection and jumps back to Kobol to await the return of Starbuck. Back on Caprica Starbuck has been shot and is under medical care in a strange looking and sounding hospital. Caprica Boomer returns to Helo, and helps him lead a raid on the hospital where Starbuck is being held which leads to a chilling discovery.
Episode 6 - Home, Part 1
President Roslin's fragile coalition is put to the test and Adama continues to struggle with the betrayal of his son. Starbuck, Helo, and Boomer return from Caprica and set out for Kobol to find the tomb of Athena.
Episode 7 - Home, Part 2
Adama heads to Kobol to find Roslin, reassembles the fleet and heads down to Kobol himself with most of the key characters (Apollo, Starbuck, Zarek, Billy, Tyrol) to find the presidential party in search of the "tomb of Athena" which allegedly leads to Earth. The two groups meet, Adama and Roslin make their peace and continue on their expedition. Apollo hangs out with Starbuck , Zarek tries to keep his cronie off plotting with Boomer who is unaware of tension building between Helo and Tyrol. On arrival at the tomb, a showdown between Boomer, Adama (and Zarek's chum) occurs but the key characters of the group enter the tomb afterwards and find themselves transported to Earth.
Episode 8 - Final Cut
A reporter is sent to Galactica to chronicle the harsh realities of life during wartime.
Episode 9 - Flight of the Phoenix
Sharon informs Adama that the Galatica has been infected with a computer virus that is tampering with the basic ship functions and is scanning them for weaknesses. All this is preparation for a major assault by the Cylon Raiders. So with the clock ticking, Adama must pull himself together and decide whether or not she can be trusted. Meanwhile the flight crew is hard at work on a new fighter named The Blackbird.
Episode 10 - Pegasus
The Galactica crew is overjoyed when they encounter another Colonial battlestar, the Mercury-class Pegasus. The elation fades when Admiral Cain takes command of the military. The Galactica crew learn about the harsh practices onboard Pegasus. The tension boils over when Helo and Chief Tyrol learn about Lt. Thorne, the "Cylon Interrogator."
Episode 11 - Resurrection Ship, Part 1
The Battlestars Galactica and Pegasus face a new Cylon target, a "Resurrection" ship, while a power struggle threatens to result in all-out war among the human fleet.
Episode 12 - Resurrection Ship, Part 2
The struggle between Adama and Cain is temporarily put on hold while the two Battlestars combine forces for an assault on the Cylon Resurrection Ship. Apollo faces a momentous decision and Baltar takes an interest in Gina, the Cylon prisoner.
Episode 13 - Epiphanies
As President Roslin nears death, Dr. Gaius Baltar must be groomed for the worst case scenario. Roslin orders the death of Sharon's unborn child. A sabotaged Viper leads to an investigation that uncovers a movement of people seeking peace with the Cylons. Baltar discovers an anomaly in the (Sharon's) fetus that may cure Roslin's cancer.
Episode 14 - Black Market
The Pegasus' new commander (Fisk) is murdered by black marketeers, and Lee is charged with solving the crime. Roslin seeks to stamp out the black market by introducing new trade regulations.
Episode 15 - Scar
A crazed Cylon Raider attacks the Colonial fleet in a series of hit-and-run attacks. Overworked Viper pilots must defend a mining operation from this Raider that they nickname "Scar."
Episode 16 - Sacrifice
A woman takes hostages on Cloud Nine and demands that Adama turn over Sharon in order to avenge her husband's death at the hands of the Cylons.
Episode 17 - The Captain's Hand
Apollo assists in the search for a missing Raptor team as the new commander of the Pegasus grows increasingly unstable.
Episode 18 - Downloaded
After Boomer's consciousness is downloaded into a new body following her death, she struggles with her new life on Cylon-occupied Caprica. Number Six is asked to help Boomer adjust to her new role in Cylon society.
Episode 19 - Lay Down Your Burdens, Part 1
Chief Tyrol is plagued by terrifying dreams and seeks the aid of a priest. Kara leads a rescue mission to Caprica. Roslin and Baltar go head-to-head in a presidential debate that changes dramatically with the discovery of a habitable planet.
Episode 20 - Lay Down Your Burdens, Part 2
When Baltar wins the vote, Roslin considers stealing the election because she believes he is a Cylon collaborator.
Descriptions adapted from: tv.com