"The central promise of the religion, everything Origin's followers devote themselves to, is a lie."
— Orlin
The Prior plague is unleashed upon Earth and quickly spreads to the unsuspecting public after a member of SG-6 is used as a carrier. Carter and Lee attempt to develop a means of fighting the Priors and receive some unexpected help in the form of Orlin, who returns to mortal form as a child in order to reveal the truth about the Ori and lend his assistance in stopping them.
"The Fourth Horseman, Part 1" provides examples of the following tropes:
- Alien Non-Interference Clause: Orlin reveals that the Ascended Ancients' long-established non-interference policy is a direct consequence of their original schism with the Ori millions of years ago. The Ancients explicitly adopted this policy to prevent themselves from going down the exact same road of corruption as their brothers and sisters.
- Attack Hello: Mitchell and Daniel are jumped by the Sodan upon returning to their planet in search of assistance.
- Awful Truth: The Ori's promises of enlightenment and ascension are revealed to be utter lies. The Ori are instead using worship of them as a power source.
- Brought Down to Normal: Orlin returns to human form in order to help Carter without violating the ascended non-interference clause. As if that wasn't bad enough, he has to take the form of a young child in order to retain as much Ancient knowledge as possible.
- Call-Back: Lam snarks at Landry that he wanted to be a general when he starts to complain about the crises piling up, referencing a similar comment that he made to her about wanting to be a doctor in "The Powers That Be".
- The Cameo: General Hammond makes a brief appearance giving a speech that's attended by the infected Lt. Fischer.
- Casual Danger Dialog: After Daniel and Mitchell are jumped and held at spear-point by the Sodan:Daniel: We still fine?Mitchell: Yeah, we're fine.Daniel: Good. Just checkin'.
- Civil War: Landry notes that the Free Jaffa Nation could well tip into this before long due to the disagreements about Origin among its members.
- Contamination Situation: Pretty much every effort to contain the plague fails due to its long incubation period, and by the end of the episode the President orders all borders closed.
- Continuity Nod
- Carter and Lee are working on an anti-Prior weapon using the data they were able to get from Khalek.
- The destruction of Kallana remains a point of contention among the Jaffa who don't want to follow the Ori.
- Lam mentions that the plague is almost identical to the one seen in "The Powers That Be".
- Carter makes several references to Orlin's previous habit of constructing complicated bits of technology out of Noodle Implements.
- Mitchell demonstrates proficiency in the fighting style of the Sodan.
- Crazy Enough to Work: Lampshaded:Carter: You said yourself it would be risky.Mitchell: Yeah, yeah. I know I said that, but the more I'm hearing, the more it's crossing over into "crazy". (the others look at him) Don't get me wrong, I like crazy. That's why I asked for this job.
- Dude, Where's My Respect?: Poor Dr. Lee.Lee: You know, I keep telling myself that one of these days, I'm gonna do something that actually earns me a little respect around here.Daniel: We're all waiting for that day, Bill.
- Expecting Someone Taller: Landry and Lam are somewhat nonplussed to meet Orlin, since he's a barely-adolescent boy and the file on him states that he and Carter had an "intimate" relationship.Carter: He didn't look like that. He was...Lam: Taller?Carter: He was a grown man.
- Face–Heel Turn: Gerak converts to Origin and tries to force a motion on the High Council to make it mandatory for all Jaffa.
- The General's Daughter: Both Mitchell and Col. Barnes chew out Lt. Fischer for making a sexist comment about getting a sponge bath from Lam, though to be fair Barnes does make the point that it's an inappropriate way to be talking about any woman on the base.
- Gods Need Prayer Badly: Orlin reveals that the Ori somehow drain energy from those who worship them and have no intention of fulfilling their promise to ascend their followers since that would mean they'd have to share the power.
- If You're So Evil, Eat This Kitten!: The Administrator tells Gerak to demonstrate his loyalty to the Ori by wiping out the Jaffa who oppose him.
- The Jailbait Wait: Orlin tells Carter that his feelings for her haven't changed, and while he understands the taboo that would prevent them having a relationship while he's in a much younger body, he's hopeful they can pick things up where they left off in a few years. Carter looks faintly disturbed by the whole discussion.
- Loophole Abuse: Orlin takes the form of a child in order to retain his ascended memories, which are too much for the adult human brain to cope with.
- Magic Versus Science: As Orlin puts it, this was essentially the original schism between the Alterans and the Ori for lack of a better term.
- Metaphorgotten:Lee: Trying to achieve something as specific as isolating one part of the brain, that's like... I mean, it's like, uh... it's like trying to... do something that's impossible.Carter: Good analogy.
- My Skull Runneth Over: It's revealed that even as a child, consciously trying to hold onto his ascended memories while human is causing parts of Orlin's brain to shut down, making him forget if he's already done something or not.
- Neglectful Precursors: Bra'tac mentions that part of the reason the promises made by Origin are so tempting to many Jaffa is because they now know the Ancients aren't going to help them ascend.
- Not Listening to Me, Are You?: Lee spends several minutes talking to Carter before realizing that she's wearing earphones and hasn't heard a word he's said.
- O.O.C. Is Serious Business: Daniel correctly points out that Orlin is openly violating the entire Ascended Ancients rule-book. They should have stepped in to stop Orlin from descending, or at the very least wiped his memory upon descending (as they previously did with Daniel). Orlin concedes he's just as confused, but he thinks a majority of his 'colleagues' turned a blind eye because of how serious the Ori threat is. They needed someone to step out of line to warn the lower planes about the truth of the Ori's origins and real intentions.
- Pitiful Worms: Daniel mentions that most of the ascended beings he's familiar with view humans in the same way that humans view insects.
- The Plague: The Prior plague reaches Earth after an SG team member is used as a carrier, and spreads throughout most of the United States within a matter of days.
- Pun: While Carter and Lee are trying to develop an anti-Prior technology based on sound waves:Lee: All we've got is a bunch of uncorrelated data and a theory. I admit, it's a pretty sound theory... if you'll pardon the pun. (bursts out laughing) Ultrasonic waves!
- Science Is Wrong: Orlin claims that the Ori took this view towards the scientifically-inclined Alterans back when they all belonged to the same group of advanced humans, eventually attempting to wipe them out as their religious fervor grew.
- Touched by Vorlons: Gerak is turned into a Prior.
- Typhoid Mary: Lt. Fischer is used as a carrier by the Prior to bring the plague to Earth, and doesn't become ill himself since he wouldn't be able to spread the disease as efficiently if he died from his own symptoms.
- Unwitting Pawn: Orlin reveals the Priors have no idea that the Ori don't ascend their followers and are merely used as pawns to do their bidding, prompting Daniel to suggest they could try turning a Prior to their own side with the truth.
- What Is One Man's Life In Comparison?: The Administrator pulls this on Gerak when he's reluctant to kill his fellow Jaffa in the name of the Ori.
