When a group of refugees board Galactica with a curable illness, it becomes very mysterious when they begin to die after taking the injection they do not believe in.

When the head of the fuel production plant holds the fleet hostage by providing unsafe fuel, he's arrested for treason, which results in the Chief sent to the ship and held responsible for getting the plant back up and running.

Besides having to deal with the loss of Kara Thrace, the crew of the Galactica is also charged with the security surrounding the trial of Dr. Baltar and his defense attorney, who is replacing one lawyer that has already been killed.

Baltar goes on trial for his life, and it affects everyone from Lee on up to the president.

During a seemingly hopeless battle, Kara Thrace appears in a pristine Viper, claiming that she has been to Earth and can lead the Fleet there -- but the terminal Laura Roslin believes otherwise. Meanwhile, four of the so-called Final Five are forced to tackle the repercussions of their newfound nature, while Gaius Baltar discovers he has a commune of nubile female cultists who believe him to be an agent of the "one true God".

Starbuck (Katee Sackhoff) and Laura Roslin (Mary McDonnell) face off with a gun between them, Lee Adama (Jamie Bamber) says goodbye to the Galactica, violent conflict breaks out between the seven models aboard Cylon Basestar, and Gaius Baltar (James Callis) is directly pulled into the "Final Five" situation.

A shaky alliance between the Colonial Fleet and Cylon rebels is put to the test when the two groups combine efforts to take out a Cylon "Resurrection Hub". Meanwhile, Athena loses control when Hera becomes unnaturally drawn to Natalie.

The Hybrid keeps jumping the basestar, and Roslin has visions. The three, D'Anna, is un-boxed. Humans and Cylons engage in a battle to destroy the Resurrection Hub. Baltar is badly wounded and Roslin has to decide whether or not to help him.

The Fleet refuses to cooperate with Cylon engineers and Adama decides to put an end to it. He sends the Quorum a message stating that unless they wish to join Zarek in the cell, they get their people in line. However, Gaeta has already freed Zarek with the help of other would-be-revolutionists. They allow Zarek to leave Galactica on a raptor. He lands on Colonial One and enters a gathering of the Quorum. Meanwhile, Anders gets ambushed and captured. At the CIC, Gaeta cleverly manipulates everyone else until the time is at hand for the revolution to begin.

Roslin is on board a raptor en route to the Rebel Cylon Base Star, but Gaeta has ordered to engage and destroy it. The marines have used a stun grenade on Adama and Tigh. Roslin manages to get a wireless transmission out for everyone to hear and learn what Gaeta has done. She and the others barely make it to the Base Star, where she convinces Tory and the other Cylons to use the Fleet as a cover in order to prevent Gaeta from attacking it. She also believes that Adama will be able to save the rest of the Final Four. Temporarily safe, the Rebel Cylons start voting on whether they should just jump away and count their losses. Adama is brought before Gaeta and Tigh is thrown to the brig with Caprica Six, Anders, Athena, Helo and their child Hera. Zarek comes aboard Galactica on Colonial One with the Quorum. At Gaeta's insisting, Zarek agrees to put Adama on trial for treason, with Romo Lampkin assigned as his defence lawyer. There's just one problem: The Quorum refuses to recognize Zarek's authority. However, Zarek is hell-bent on following his "The truth is told by those left standing" maxim. Only Tyrol, who's alone, and Lee, whose with Starbuck, are Galactica's last hope.

The Colony is located sitting on the edge of a black hole with only one point of entry in what is a very large debris field. Knowing that the Galactica has reached the end of the line, Admiral Adama asks for volunteers to mount a raid on the Cylon base, realizing it may be the ship's last jump. In flashbacks prior to the Cylon invasion, key moments in the lives of several of the crew are presented. These include include the first time Kara Thrace and Lee Adama meet; Admiral Adama's struggle with his decision to retire from the colonial fleet; Gaius Baltar's relationship with his father and his developing relationship with Caprica Six; and Sam Anders discussing the motivation behind his professional sports career.

A stripped down Galactica attacks the Cylon's Colony ship in the hopes of rescuing Hera. The meaning of the shared dream in the Opera House on Kobol is revealed. Sam Anders is moved in his Hybrid tank to the CIC in the hopes that he will be able to assist the combatants. Their mission complete, Admiral Adama orders Starbuck to pick a destination - any destination - to which the ship can jump to get out of there. With that, the meaning of the tune and the musical notes are explained. Having successfully jumped, the old Galactica has truly reached the end of it's life. A planet capable of sustaining life is found and Lee Adama makes a radical proposal for the future of humankind. In a flash forward far into the future, Hera's importance to the human race is revealed.

As conflicts reach a climax, Cylons and humans face a stark choice.