Still in the underground caverns of Avalon, the team have to face several challenges to prove that they are worthy as well as tests to in knowledge to allow them access to the knowledge of the Ancients.

After successfully completing the quest, and finding riches, they hadn't dreamed of, they find an Ancient device and an Anceint book, which tells the team of a group of ancients who moved along to another galaxy.

However, when Daniel, and Vala activate the device they are transported to another planet, and things get out of hand when the people of the village try to burn Vala alive.

After being re-iterated by a prior of the Orii, Vala and Daniel are taken to meet the Orii, where they learn the truth about the Orii. They learn that the Orii are in fact Ascended beings, who unlike the Ancients use their power for their own means, and have zero tolerance for any disbelievers, and destroy all those that oppose.

Things become worse, when the Orii are made aware of the humans presence in the milky way, and they soon sends Priors to convert the people to Origin, and to kill any disbeliever.

After realising that the Ancient device mixed with the Goa'uld armlets has bound the two together, Vala, Daniel, Teal'c and Mitchell must undertake a dangerous mission to break the ties that bind them, however things are made harder when they must go into Vala's past.

Concerened about a planet who trust Vala, Mitchell takes SG1 to one of Vala's previous homeworlds, to discover that she used the peoples belief that the Goa'ulds are gods and a device to make them follow her, and serve her as her god, even when the Tok'ra had removed the Symbiote.

When the people discover the truth, they imprison her, and things are made worse when a Prior of the Orii visit the planet and command that the people believe in the Orii or suffer the consequences. However, soon after Daniel tries to get the Orii to leave, the people misteriously start to become sick, and Vala is the only one that can save them.

When they realise that the fight against the Ori has started, General Landry recalls Sam from Area 51 in the hopes that her new weapon will destroy a force field placed on a Free Jaffa planet by the Ori.

When Peaceful negotiations fail, the weapon is used, but instead of destroying the force field, it enhances it, and soon, the entire planet is threatened. The true motives of the Orii are soon revealed, and a sacrifice must be made to rectify the proble, but who will be the one to make the sacrifice?

When a Jaffa turns up dead on Earth, relations are strained between the new Jaffa Nation and the Tau'ri, as each try to find Ba'al who has taken refuge on Earth, however things are made harder, when they discover, that anticipating this, Ba'al has set a Naquadah bomb somewhere on the planet that could kill hundreds of people.

In the meantime, Teal'c must try and force Garak to remove his forces from Earth.

Whilst on a Planet, Mitchell is gravely wounded in a battle with the Sodan warrior Volnek, and is taken captive and nursed back to health only to be prepared for a fight to the death for spilling Sodan blood.

The team discovers that Anubis left behind a mutant human, Khalek, who has Anubis' genetic memories and the potential to ascend and attack the Ancients. When Daniel realizes the threat that Khalek poses, Mitchell and Teal'c go to Dakara to seek more information about him.

In the meantime, Woolsey wants the chance to study Khalek, regardless of the danger that he posses.

On Dakara, Garek is visited by a Prior, who wishes him to accept Origin as his religion, and Garek, having read from the book of Origin tells him that he has seen the light, and that Origin is all that the Jaffa are looking for, and that in time the Jaffa will all see Origin as their true faith.

At the SGC, SG6 come back from an undercover mission to an Ori-converted planet, who confirms that this is the third planet converted in the past month. In the mean time, Colonel Carter, tells General Landry on a new project, that she and Dr. Lee have been working on, a device which could inhibit the Priors ability to use their powers, created thanks to the fiasco with Khalek.

Back at Dakara, Garek brings the proposal to the high council, and says that it be made compulsary for all Jaffa to follow Origin. However, even Teal'c and Bra'tac fail to convince many of the high council.

In the meantime, Colonel Barnes, of SG6 becomes ill, and Lam discovers that he has been infected with the Prior Plague. Landry has SG6 braught back to the SGC, however they discover that it has been too late and the Prior Plague is now spreading throughout the population.

Sam and Dr. Lee back at the lab, try to make the device, however getting nowhere, Lee goes to get coffee, and Sam who beleives she is alone finds herself with a young boy who claims to be Orlin. Lam confirms that he is human and they soon realise that he is telling the truth.

Orlin tells the SGC that the Ori do not give their followers Ascension, and instead drain power from their beliefs. With every human that starts believing in the Ori, they gain slightly more powerful, although they require a lot of human beliefs to gain even the slightest amount of power.

On Dakara Garek meets with a Prior who tells him to destroy all those that do not believe.

On the base, Orlin uses what remains of his ancient knowledge to try and construct a viable cure for the Prior plague. However, he shows signs of memory loss, and on further inspection they learn that his mind is trying to hold on to as much Ancient knowledge as they can, however this will result in permanant brain damage.

In the meantime, Dr. Lee completes the anti-Prior weapon, and they choose to travel to to the Sodan planet in an attempt to test the weapon, and capture themselves a Prior. On the Sodan homeworld,

Whilst Teal'c holds his grounds on Chulak, Garek warns him that he will not hesitate to destroy them unless they bow down to the Ori, and believe in Origin. Garek returns to Dakara, and is once again visited by a Prior who takes his mind to Celestis to quell his doubts.

He returns to Dakara to reveal that he himself has turned into a Prior.

With the Prior Plague quickly becoming a global pandemic SG1 must find a cure. Garek in the meantime now a Prior tells the Jaffa High Council about the powers he has received as a Prior. However, Teal'c and Bra'tac are far from convinced and make a plan to stop Garek.

On the Sodan planet Mitchell meets with Lord Haikon and tries to persuade him that the Ori are not to be trusted, however his efforts go un-needed as Hiakon and Jolan tells him that they have already decided to renounce the Ori.

Orlin continues to have key information slip away from him, and so Sam tries to have him teach her, however he fears it is far too complicated and she would not be able to understand.

On Dakara Teal'c and Bra'tac meet with Cha'ra and convince him to help mount a defense against Garek's forces who will soon destory all those that oppose the Ori.

Whilst on an alien planet, Mitchell is accused of a brutal murder, and his own memories appear to have become altered, and confirm that he did indeed commit the murder.

It is upto SG-1 and the SGC to protect Mitchell while they prove his innocence, and find the real killer.

When Orin the Ancient that fell in love with Sam ascended once again, he was instead returned to the body of when he was a child. He once again meets Sam and the SG1 team.

During a tour of the Beta Site for a group of foreign delegates, things become dangerous when SG-1 encounters an insidious new threat. The Pentagon orders SG-1 to escort diplomat Richard Woolsey and three representatives from the International Oversight Advisory on a tour of the Gamma site. At that off-world base, the British, French and Chinese diplomats meet scientists who are studying alien bugs that might have been created by the Priors. Meanwhile, Daniel and Mitchell speculate that the real purpose of this tedious mission is to placate the Chinese, who are frustrated by American refusals to share stargate-related military technology.

Suddenly, the voracious bugs escape containment and kill a scientist. As the critters multiply, taking over the 'gate room and swarming the base, Col. Pearson, the base commander, sends the IOA representatives outside for their own safety. Guarded by SG-1 and two airmen, the group struggles through dense forest toward an uncrewed research station, where they can wait until Pearson and his team defeat the infestation.

They don't get far before the bugs — which burrow underground and have highly sensitive hearing — burst from beneath the feet of an airman and devour him alive. The survivors race to a nearby cave, where the rock walls can protect them … for a while.

During a long, tense night in the cave, Mitchell tries and fails to radio Pearson while Daniel, true to form, squeezes in some high-stakes diplomacy with the Chinese delegate.

In the morning, the second airman dies horribly, the victim of a single insect that laid its eggs within him. Determined not to suffer the same fate, Mitchell and Teal'c risk returning to the base, hoping to find F-302s with which they can fly everyone to safety. As they approach, however, a massive explosion rocks the ground. The Gamma site's self-destruct has gone off, leaving nothing but a smoking ruin.

With the F-302s destroyed in the explosion, Mitchell and Teal'c return to the cave. There, Carter explains that Gen. Landry, on Earth, will have to follow standard procedure to defeat the infestation by sending the Odyssey to the planet with a neurotoxin that kills bugs and humans alike. Only hours remain before the Earth ship arrives; when it does, it will unwittingly annihilate the group from orbit unless they can reach it with the news that they're still alive.

The planet's radioactive ionosphere will block the team's usual transmitters, but the research station might have the communications equipment they need. To get there, however, SG-1 must lead four frightened diplomats through five miles of bug-infested forest.

Mitchell and Sam are shifted to another dimension, and are unable to interact with anyone but each other. However, Daniel realises what has happened to them and looks for a way to interact with them.

In the meantime, Teal'c leads Major Hadden and SG9 to the homewold of the Sodan, however, when they get there they find the Jaffa have been savagely attacked, and the leader of of the Sodan is near death, and without a symbiote. However, before they can take him back to the SGC and give him tretonin to save him, they discover that their only transport to the stargate has been damaged and the only way back to the stargate will take several months by foot.

Teal'c soon sets out to search for Volnek, the Jaffa who betrayed his fellow Jaffa and attacked them, and Mitchell soon uses his invisibility to his advantage and sets out to track Volnek down himself.

Using the ancient communication, a mysteriously pregnant Vala is able to comunicate through Daniel, and tell General Landry about an impending Orii attack.

They learn that she has been living in the Orii galaxy for the past few months in peace, and has married Tomin, the man who has given her shelter, but has mysteriously become pregnant without a sexual union. They learn about a sect of Anti-Orii who are undertaking a mission to destroy the Orii's new fleet before they can leave.

However, with Tomin a deep devotee of the Orii, will Vala stay safe, and will the Orii turn Tomin against his wife?

With the Anti-Orii movement undertaking a dangerous mission to destroy the new Orii fleet, will they be successful, or will the Orii be able to stop them, and get to the Milky Way where they can start an invasion in this side of the Galaxy