Operation: Z.E.R.O. (Zero Explanation Reveals Origins) is The Movie to the cartoon Codename: Kids Next Door.In this movie, an old evil named Grandfather is recommissioned by his son, Father and takes control of the world. He turns everyone into Senior Citizombies and soon, the KND begins falling apart as one by one, all of its operatives turned into Senior Citizombies... Including Numbuh 2, 3, 4, 5, 362, 60, and 86...It looks like Numbuh One is going to have to turn to a hero... A hero named Numbuh Zero...
Operation: Z.E.R.O. provides examples of the following tropes:
Adults Are Useless: For a show that usually ran on this trope, this movie provides the ultimate subversion in that Numbuh One's father, who turns out to be the fabled Numbuh Zero, ultimately ends up saving the day.
Grandfather. And HOW. He practically destroys the KND and succeeds in retaking control of the world like before but worse. Yet instead of kicking up his heels like most would do and relaxing with some tapioca he then focuses his attention on destroying a last Hope Spot, the book of KND which led to his defeat the last time. Even when one of his own sons show some resistance to their family's evil ways he decides to zombify him instead of just leaving him be.
And it was by restoring Grandfather's memory that Father showed another moment of Genre Savvy. To steal the memory-restoring device from the museum, he sent two groups of villains: one to the museum and one to assault the moonbase as a distraction. Almost all KND operatives believed the museum raid to be the distraction. Later, when Father assembled the other villains so they'd see him using the device on Grandfather, he not only expected Numbuh One to show up but also wanted it to happen. It turns out the device couldn't be activated without boogers from an active KND operative and, since Father's and the Delightful Children's involutary inductions were already over, he needed to take it from somebody.
Fanboy: Numbuh 101 is this for Sector V. He geeks out at being saved by the team, has various items that belonged to the members of Sector V from past episodes and even has all their autographs!* But not in red.
Fighting Your Friend: Numbuh One is forced to fight the rest of Sector V because they've all been turned into Senior Citizombies. Naturally, he doesn't give it his all and quickly finds himself on the losing end, until Sector Z came to help.
Also, Numbuh 5 says to Numbuh 1 that she's glad she'll never have to fight him, since he's crazy. Sure enough, he does end up fight her, along with the rest of his zombified team
Freudian Excuse: Father's got a damn good one. It seems that being raised by a horrendous asshole who turns people into zombies doesn't exactly inspire you to be a good person.
Funny Background Event: When Father was explaining his plan to Numbuh One, you can see Grandfather (not yet recommissioned) trying to walk off stage, only to be stopped by the open palm of someone off-screen. Twice.
Informed Ability: In the beginning, Numbuh One says he knows a fighting technique that he can use to defeat four enemies at once which he's "always wanted to try." This becomes sort of a Brick Joke; near the climax of the special, he's attacked by his Brainwashed teamates and mentions it again, but says he did not want to try it on them. He never actually gets a chance to try it, so we can only assume this is true.
Numbuh One boasts that he has a super cool new move that can take down four enemies at once. Unfortunately the first time he tries to use it, the villain had just succeeded with their goal and called a retreat before he could even attack one of the four guys around him. Then he thinks to use the move a second time later in the movie, but because it's against his zombified team, he hesitates and ends up defending the whole fight until Sector Z comes to help.
There's also Father's little burst of raw anger against Grandfather (during which even he was cowering in fear), but then decides Screw This, I'm Outta Here! when he gets to depressed to fight. This could really be seen as Trope Codifier.
Nice Job Breaking It, Hero: It was because of Numbuh One's rash decision to stop the villains' convention that Father managed to recommission Grandfather. He got a Heroic BSOD because of it.
Nigh-Invulnerability: Grandfather. They crash the Moon Base on him and he dusts it off like it was nothing! (To be fair, the KND didn't expect it to hurt him; it was part of a plot to make him lose his memories, which did work.
Oh Crap: The villains have this as a collective reaction when Grandfather declares he was going to age-ify everyone who's ever been a kid.
Grandfather himself during the climax.
Grandfather: (After grabbing the Book of K.N.D from Numbuh 0) Yes! With this my victory is complete! Nothing will be able to stop me now! Not even... (looks up and in a meek voice)... A gi-hugic flaming moon base?!
Our Zombies Are Different: The Senior Citizombies. They're always old no matter how old they actually were before they're transformed, but they're still very capable fighters and are able to age-ify others. They also retain their basic memories and personalities, although they are totally subservient to Grandfather.
Rapid Aging: His primary ability is to age everything he touches, though not only does he age them rapidly, he turns them into his "Senor Citizombie" minions. All individuals he ages gain the ability to do it to others.
And lastly, there's the fact that The Delightful Children are actually the legendary Sector Z! Numbuh One has a temporary freak out due to this:
Numbuh One:The Delightful Children from Down the Lane are the missing Kids Next Door members of Sector Z! Dad, the Delightful Children from Down the Lane are the missing Kids Next Door members of Sector Z!
Roaring Rampage of Revenge: Father becomes even scarier when he's mad, with his fire powers becoming more powerful than ever before and seemed like it was about to match Grandfather's. Subverted, he doesn't even do anything after shouting and simply goes back to his house to eat some ice cream.
Grandfather: Even you, my son, are zero compared to me. Zero: That may be, but my son is Numbuh One!
Squick: In-universe example: when Numbuh Two (with Numbuh 362 and Numbuh 86 behind him) contacts Numbuh Three and Numbuh Four, he is greeted with the two of them age-ified and holding hands, declaring they've found each other before kissing on-screen! All three of them are horrified by this, and Numbuh 362 immediately orders to end transmission.
Tempting Fate: Numbuh Three wishes Numbuh Five was in Sector V's treehouse when she and Numbuh Four were there. She shows up the next second as a Senior Citizombie
Throat Light: All the characters turned into Senior Citizombies have streams of light pouring out of their eyes and mouth during the transformation.
Title Drop: Though the phrase "Numbuh Zero" is thrown around quite a lot, there are two instances where a character exclaims "Zero!" can count:
Grandfather: I'm giving you three seconds to go to your room, or else! Three... Two... One... Numbuh Zero:Zero!