The episode begins at Pop and Cub's house, where Pop is seen giving a milk bottle to Cub. However, the bottle slips from his hands and lands in the middle of the street. As Cub retrieves his bottle, he is unaware that an approaching vehicle is coming. Pop reacts to this quickly, and pushes Cub out of the way, only to have Cub get shredded apart by a street cleaner driven by Lumpy. Because of this, Pop yells out "NOOOOOO!" into the skies.
Later, inside a garage, Pop is seen building something. When he's done, it turns out to be Cub, now a robot who is as tall as their house (now called Cubtron). Pop and Cubtron decide to go into town. While walking around, Pop spots an arts and crafts store, so Pop tells Cubtron to stay, while he gets something. While waiting, Cubtron decides to entertain himself. First, he rips open a school bus and pulls out Cuddles, who is laughing. Cubtron then proceeds to press Cuddles' body against a building, killing him and spreading his blood. Cubtron then picks up a screaming Petunia and Giggles from the school bus, and they suffer the same fate as Cuddles. As Pop comes out of the arts and crafts store with a box of crayons, he notices what Cubtron just did. It turns out that Cubtron used the bodies of the victims like crayons to make a drawing of himself holding his father in his palm. However, Pop does not approve of this, so Cubtron picks up Lumpy's street cleaner to erase the drawing. But this also causes Lumpy to fall out and break his legs in half when he hits the ground. With his legs broken, Lumpy proceeds to crawl away.
As Pop watches TV on a flying Cubtron, the TV is displaying a monster attacking the city. The monster and Cubtron stare at each other menacingly, and charge at each other, indicating they're about to attack. However, Cubtron instead jumps onto the monster's arms so it could take care of him. The monster is happy to oblige, and tears off a container on a water tower to feed him (as if with a milk bottle). Suddenly, Cubtron begins crying. His loud crying causes nearby windows to shatter with one of the shards cutting a crutch-toting Lumpy's head in half. Pop is unsure on what to do, so the monster decides to repeatedly pat Cubtron's back. This causes Cubtron's tummy to rumble and beep rapidly, indicating something big is about to happen. As Pop embraces for impact, Cubtron lets out nothing but a short burp. Just when everything seems fine, Cubtron begins vomiting. His acidic vomit splashes on Mime, Toothy, and Disco Bear, causing the bodies of all three to be reduced to skeleton and making their heads burst. The vomit is also strong enough to burn down and destroy all the buildings.
Sometime later, the entire city is destroyed, and amidst all the damage, the only thing not destroyed is Pop and Cub's house. The monster has resorted to becoming Cubtron's adoptive mother, as it wears an apron and has just finished making a batch of cookies. As Pop asks Cubtron to let him sit on his lap, Cubtron proceeds to do so, unknowingly crushing his father offscreen.
"Cubtron Z" contains examples of:
- Art Shift: This is the only HTF episode ever that was animated in stop-motion, as opposed to being animated in Adobe Flash like how the show is done normally.
- Bait-and-Switch: When Cubtron and the Monster run towards each other it almost looks like they are going to fight but instead, Cubtron leaps into the monster's arms so the monster can love him.
- Big "NO!": Pop does this after Cub dies.
- Bittersweet Ending: The city turns out to be destroyed but Pop and Cub's house is safe along with the Monster helping Pop take care of Cubtron being Cubtron's mother figure, unfortunately Cubtron unknowingly crushes Pop when he asked him to sit on his lap. However, at least Cub (as Cubtron) survives in the episode.
- Female Monster Surprise: The Kaiju sea monster. You'd at first think the monster was a male as it has a male voice but at the end of the episode, the monster is revealed to act like a female showing to wear eyelashes and an apron.
- Heel–Face Turn: The Monster becomes a nice parent figure after it met Cubtron and later is shown living with Pop and Cub.
- I Just Want to Be Loved: The monster is first shown to be evil but after it interacts with Cubtron it's shown to be a nice loving helpful monster showing that it might just wanted to be respected.
- Karma Houdini: Cubtron and the Monster suffered no comeuppances for destroying the city.
- Karmic Death: Lumpy has killed Cub in the beginning of the episode, and once Cub is revived as Cubtron he kills him later on.
- Monster Is a Mommy: There's a fish-like kaiju monster that acts motherly towards Cubtron upon meeting him. Later in the end of the episode, it makes cookies for both him and Pop.
- Ship Tease: The monster looks like a housewife at the end of the episode, which means it possibly wants to be Pop's new wife.
- Skyward Scream: Pop, after seeing Cub getting killed by Lumpy
- Small Parent, Huge Child: Cubtron is a robot who is way bigger than Pop
- Stripped to the Bone: Cubtron's acidic puke ends up splashing on Mime, Toothy, and Disco Bear; leaving nothing but their skeletons behind, while also causing their heads to blow up.
- They Killed Kenny Again: Cuddles, Giggles, Petunia, Pop, Mime, Lumpy, Toothy, Disco Bear, and Cub (but he was revived as a robot) die.
- Tertiary Sexual Characteristics: The ending in "Cubtron Z" shows the monster Is wearing an apron and having eyelashes, implying it's feminine side.
- Throw the Dog a Bone: Cub survives.
- Too Dumb to Live: You’d think Pop would know better than to let a giant robot sit on his lap, even if it is his son…
