The Russian SG team returns through the gate with a captive from the planet they have been visiting. After O'Neill and General Hammond both yell at them for a bit for violating procedure, they agree to listen to the man's story. His name is Alebran and he claims that his people are being experimented on by the Goa'uld Nirrti. He begs the SGC for help against her, and then dissolves into a puddle of water because of a failsafe Nirrti built into his very DNA.
Figuring that she is continuing her experiments to advance human evolution and thereby create the perfect host, SG-1 decides to investigate. When they get there, however, they find that Alebran's horribly mutated and hideous people do not want to be liberated. They consider Nirrti to be their savior (from a plague that ravaged their civilization) and worship her as a God. They have also developed superhuman abilities - telekinesis and telepathy in particular - and have SG-1 and Col. Ivanov of the Russian team locked up in short order. All the better for Nirrti to experiment on.
One at a time, Nirrti plugs the captured team members in to her DNA scanning machine and futzes with their biology. Ivanov is first, followed by Carter. When it comes around to Jonas's turn, however, Nirrti apparently sees something she likes. He wakes up in her personal quarters, where she reveals that he is far advanced in his evolution, and offers him unlimited power if he will join her. When he systematically refuses to put out, she tosses him back into his cell, just in time to see Col. Ivanov dissolve into a puddle of water.
While Jonas was away refusing to be seduced, O'Neill has been trying to convince the mutants that Nirrti is bad news. He argues that if they just read her mind (which happens to be forbidden) it will confirm everything he's said. It needs to be quickly too, because Carter is beginning to look pretty moist. They ignore him and drag him off to have his DNA manipulated. He trades his usual banter with Nirrti for a while, and again tries to get the locals to look into her mind. This time, they do - and realize that she's an evil monster, just as O'Neill has been saying this whole time. With their telekinetic powers, they rescue O'Neill and kill Nirrti - which is bad, because Carter is going to melt into a pile of goo any minute now.
Fortunately, they also managed to steal Nirrti's knowledge of how to operate the machine. The restore Carter in short order, and then themselves, saying they will destroy the machine once they are done.
Jonas is probably fine.
"Metamorphosis" provides examples of the following tropes:
- Alien Sky: P3X-367 has two moons.
- Body Horror: Alebran and Ivanov both literally dissolve into water; Fraiser speculates that this is the result of all the body's cells breaking down at once. Also, the mutants are all varying degrees of disfigured and misshapen, with some of them having additional eyes in places that eyes have no right to be.
- Continuity Nod:
- There is now a Russian team operating out of the SGC.
- Ivanov and later O'Neill himself both bring up the fact that O'Neill let Nirrti go free after their last encounter.
- At the beginning of the episode, the team worry that Nirrti might have intended to use Alebran as a Trojan Horse, the same way she did with Cassandra.
- Curse Cut Short: Carter calls Jonas a "chickenshit" when he asks for her help in talking to a nurse he likes, but the last part of the word is drowned out by the "offworld activation" alarms going off.
- Evilutionary Biologist: Nirrti.
- Eyes Do Not Belong There: Due to Nirrti's genetic manipulation, Wodan has a third eye above his left ear.
- Hoist by His Own Petard: Nirrti is killed by the very people she's been experimenting on after they realize she's been lying to them, as her "treatments" have made them too powerful for her to control.
- I Cannot Self-Terminate: Alebran starts screaming and begging for somebody to kill him shortly before he melts into a puddle of water.
- Killed Off for Real: Nirrti.
- Mutants: Alebran's people, of the mutate variety.
- My God, You Are Serious: As revealed on the DVD Commentary, this trope took place behind the scenes. During the writing of this episode, Showrunner Brad Wright repeated suggested that episode writer James Tichenor include a seduction scene with Nirrti and Jonas. Assuming that Wright was joking, Tichenor never wrote it. Later, Wright wrote a draft of the script and added the seduction scene himself.
- Neck Snap: Wodan kills Nirrti this way after telekinetically lifting her into the air.
- Nice Job Breaking It, Hero!: It's implied throughout the episode that O'Neill feels partially responsible for the plight of Alebran's people after he honored his bargain with Nirrti and let her go at the end of "Rite of Passage". She even reminds him that she admitted at the time she wouldn't have done the same in his position.
- Psychic Powers: Some of the mutants have developed psychic abilities after being experimented on by Nirrti. Notably, Eggar is telepathic while Wodan has the power of telekinesis.
- Psychic Strangle: Woden does this to Nirrti at the end of the episode. Ultimately, he psychically snaps her neck. He also kills two of her Jaffa with a psychic strangle.
- Red Shirt: Col. Ivanov.
- Sleep Cute: Carter rests her head on O'Neill's shoulder to sleep when he orders her to get some rest, though the moment is somewhat ruined by Ivanov exploding.
- Slouch of Villainy: Jonas wakes up after his turn in the machine to see Nirrti watching him while sprawled on a chaise lounge.
- Take Me Instead: O'Neill does this when Carter is taken away, and then again when he's brought before Nirrti, not that it gets him anywhere.O'Neill: Fix what you've done to Carter, let the rest of my team go, that's all I ask. You can do whatever you want to me.Nirrti: Yes, Colonel. I can.
- Transhuman: Nirrti implies that Jonas — or possibly his people as a whole — are somewhat more advanced along the evolutionary scale than Earth humans. This never really goes anywhere, however.
- Unwanted Rescue: The mutants initially reject the team's help, as they have embraced Nirrti as their god and believe that she is helping them. This changes after Eggar reads her mind and realizes that SG-1 was telling the truth.
- We Can Rule Together: Nirrti to Jonas. He pretends to consider it for a bit before swiftly telling her to go to hell.