The episode begins with the Tallest looking at the Megadoomer Combat Stealth Mech, which is going to be given to Invader Tenn to help her conquer the planet Meekrob. They then see a pack of dangerous malfunctioning SIR units. Tallest Purple comments that anyone using those could get seriously hurt, which gives him an idea—send them to Zim!
At the conveyor belt planet where the packages are processed, a non-Irken working there is furious about how the Irkens conquered his planet and enslaved his people, forcing them to work in postage. He decides to rebel—starting by switching the labels on the packages going to Tenn and Zim. He gets no further than that before getting zapped by an Irken slave driver, but the damage is done.
On Earth, Zim is walking home, complaining about his day at school to a random boy. He has just entered his house when the package containing the Megadoomer is deposited in his yard. He then contacts the Tallest and profusely thanks them, although he never says what he's thanking them for, which confuses them. The Tallest then get a transmission from Invader Tenn, who says "It's horrible! They're everywhere!", before the transmission cuts to the defective SIRs causing havoc and dismantling her base.
Back at his house, Zim is delighted to have the Megadoomer. GIR comments that it has chicken legs, and Zim points out that it can also turn invisible. He plans to use it to annihilate Dib, although he admits that it may be "a bit much". He enters the cockpit and presses a button, but it doesn't work. GIR finds out that the machine doesn't have any batteries, and Zim improvises by plugging the Megadoomer into an outlet. He presses the button again, and this time, the Megadoomer turns invisible. GIR points out a major problem with the device—it cloaks itself, but not its pilot—but Zim brushes him aside.
Zim steers the Megadoomer out of his house (and into a few walls), before cloaking it. He tells GIR (who's handling the enormous extension cord the Megadoomer requires) to follow the sound of crashing footsteps. Zim stomps through the city, gloating about how Dib wouldn't see this coming and how confused and frightened the humans look, entirely oblivious to how obvious he is. While he's stomping and gloating, the Megadoomer comes unplugged and powers down. GIR plugs it into another cord, causing it to activate again. A policeman shoots at the Megadoomer (which has no effect), and soon after, the Megadoomer loses power again. Zim commands GIR to re-plug it in "before we're noticed"- once more oblivious to the fact that everyone has already noticed them. GIR plugs it in at the Old Folks Home (the residents ignore him).
A car stops at a red light, with the Megadoomer behind it. Zim yells at the car to get out of the way but goes unnoticed. The car moves when the light turns green, but Zim seems to think that he was the cause, and he moves on. The Megadoomer loses power in front of Chuck and Buck, who are playing with robots. GIR plugs the Megadoomer back in and eats the toy robots.
Dib, meanwhile, is brushing his teeth. He notices that the water in the toilet is rippling. Zim then arrives outside. Dib runs out to confront him. Zim gloats that Dib will be fighting an enemy he can't see.
Zim: What?
Dib: There! Your mighty Irken cloaking device cloaks the robot but not you.
Zim: Lies! Behold the Doom Cannon!
Dib: I can't. It's invisible.
Zim: But you can see me?
Dib: That's what I said.
Zim: Oh, that's stupid!
Dib: Really stupid.
Zim: You dare agree with me?! Prepare to meet your horrible doom!
Zim then begins pressing buttons, preparing to do something. We never see what it is he was going for, because at that point, the power draw goes critical, sapping all of the city's energy. All the lights go out, and the Megadoomer is once again inactive and visible. Dib begins snapping pictures, and Zim freaks out and begins pushing buttons at random. Zim panics about his cover being blown and self-destructs the Megadoomer.
Zim escapes and takes the camera from Dib, proclaiming his victory. When Dib points out that his robot exploded and he didn't destroy Dib, Zim claims that it was all a trick, and his real goal was to steal the camera. Dib tackles him, and they fight for the camera, with Dib eventually taking it back. Zim then takes his turn with the Sanity Ball, and points out that Dib had the lens cap on the whole time. Zim runs away, proclaiming his victory, and a bird snatches the camera away. Dib goes back to bed.
On the Tallest's ship, they gain a signal from the Megadoomer, saying that it self-destructed. Purple wonders if Zim was in it, and Red says "Probably not." They then get a transmission from Tenn, saying "They've eaten everything! We're all doomed!" Shortly after this, Tenn's screen is destroyed by the malfunctioning SIRs.
Tropes
- A.I. Is a Crapshoot: The defective SIRs, which destroy Invader Tenn's base.
- Angrish: Zim's complaints about his day devolve into this.
- Awesome, but Impractical/Cool, but Stupid: The Megadoomer, which requires a massive extension cord and cloaks everything but the pilot and said extension cord.
- Bait-and-Switch Compassion: The Almighty Tallest Purple suggests sending some malfunctioning SIR units to Zim. Red gasps and says in a worried tone that he will be destroyed if he gets them. The two then laugh since they hate Zim's guts. Currently the trope's page quote.
- Cassandra Truth: Zim doesn't believe GIR when he points out that Zim is still visible despite the cloaking device being on. Dib later tries to tell Zim, but is likewise dismissed (Dib does eventually manage to get through to Zim, however).
- Disproportionate Retribution: Dib humiliates Zim at Skool, so the Irken decides to kill him with a prototype mech.
- Doomy Dooms of Doom: The Megadoomer.
- Evil Is Petty: Zim receives a Megadoomer, a Humongous Mecha and the most powerful weapon in the Irken Armada. Rather than use it to attack a strategic target or fix the weapon's faulty power systems, Zim decides the best possible use of the Megadoomer is petty revenge on Dib for a bad day at Skool; to be fair, he admits using it is "a bit much".
- Large Ham: "Silence! My victory. Begins. Nooooooow!"
- Mini-Mecha: The Megadoomer is only about a story-and-a-half tall.
- Motor Mouth: Zim thanks the Tallest for giving him the Megadoomer with the following, all said in one breath in the span of about five seconds."OH THANKYOUYOUVEDONETHERIGHTTHINGMYTALLISTYOUWONTBEFORGOTTENWHENIRULETHE
UNIVERSETHANKSFORTHISAMAZINGBATTLEMECHBYYYYYYYEEEEE!!" - One-Word Title
- Pyrrhic Victory: Both Zim and Dib declare the end of the whole misadventure a victory for themselves, even though Zim lost the Megadoomer and failed to kill Dib, and Dib didn't get any photos of either the alien or robot, AND lost his camera to a random bird.
- Psychopathic Manchild: Tallest Purple is shown the new Megadoomer, and pouts like a little kid exclaiming "I don't like it!" for no reason.
- Sequel Hook: Had the show not been canceled, Invader Tenn would have developed into this, as the defective SIRs would have destroyed her base and disguise, leading to her capture.
- Shout-Out: When the Megadoomer approaches Dib's house, the water in his toilet recreates the famous "ripple" scene from Jurassic Park.
- Small Role, Big Impact: The postal slave who switches Tenn's and Zim's packages. His actions lead to the plot of the episode.
- Smart Ball: Even a moron like Zim is able to recognize the Megadoomer's cloaking device is a piece of shit if it cloaks everything but the pilot.
- Spanner in the Works: A disgruntled postal slave switches the labels on two packages, giving Tenn the malfunctioning SIRs and Zim the Megadoomer. This results in the Megadoomer being essentially wasted as Zim panics and self-destructs it, and in the total destruction of Tenn's operation as the SIRs run rampant.
- Stylistic Suck: The weird TV show the folks in the old home are watching before Gir bursts in to use the outlet for the Megadoomer. It looks like some sort of weird sci-fi cop show with a guy who's got wires sticking out of his face.
- Super-Fun Happy Thing of Doom: The Megadoomer's self-destruct button has a smiley face on it.
- This Is Gonna Suck: The Tallests' silent worried faces say it all when they realize too late that the packages for the Megadoomer and the defective SIRs got mixed up.
- Un-Evil Laugh: Zim has a bit of trouble laughing evilly in this episode. His first attempt after declaring that his victory begins now quickly devolves from a maniacal cackle to a more awkward forced laugh after he pauses mid-laugh to take a breath. He tries again while rampaging through the city, only for his laugh to be interrupted by a cough.
- With Catlike Tread: The Megadoomer would be quite stealthy—except it doesn't cloak its pilot (making him look like he's floating), needs a gigantic extension cord (which it also doesn't cloak), and is so heavy its feet sink into the ground.
