Tear Jerker / Operation: Z.E.R.O.

• Numbuh One having a Heroic BSoD after he believes himself to be responsible for Grandfather's return.
  Numbuh One: I'm no Numbuh Zero... I'm just A Zero.
• Numbuh Five's Heroic Sacrifice.
  Numbuh Five: I'll hold off Cree! You guys get back to the Moonbase NOW!
  Numbuh Three: But we can't leave you!
  Numbuh Five: You must! Before it's too late! [gets transformed]
• The rapid fall of the KND as Grandfather's Senior Citi-zombies spread throughout the world is truly the Darkest Hour for them, not helping are the visuals and music becoming significantly darker.
• There's a brief moment shortly before Numbuh One realizes who he really is and initially recommissions him where he's dejectedly looking down after Numbuh One tells him to shut up for being a distraction and a Bumbling Dad when the former is trying to save them. You can tell how heartbroken he is to hear his son get mad at him for trying to spend some quality time together.
  • In general, every time Monty was ever saddened whenever Numbuh One angrily snaps and rejects a chance to spend time becomes Harsher in Hindsight when one realizes Nigel's father is Numbuh Zero. You see in his days, Numbuh Zero and his brother didn't have such a happy childhood and lived under the (literally) tyrannical rule of Grandfather. In other words, Nigel doesn't know how good he has it, growing up with a kindly, supportive father like Monty.
• Father's failed, rather deflated attempt at confronting Grandfather, if for no other reason than the fact that it's eerily reminiscent of Real Life abuse victims attempting to confront their abusers.
• Numbuh One having no choice but to fight his brainwashed teammates, and his ultimately failed attempts to reach out to the real them until he uses Numbuh Two's upgraded Birthday Suits.
• The Reveal that the Delightful Children from Down the Lane are actually "delightfulized" Kids Next Door operatives, specifically Sector Z. Also heartbreaking is the news that their return to their true selves is only temporary, and even worse is their final words when they do inevitably revert to the Delightful Children. Numbuh One can do nothing but watch and let out a Big "NO!" as it happens before his very eyes.
  Sector Z/DCFDTL: Tell... the Kids Next Door... we miss them! AHHH!!!
• Numbuh Zero's final message to Numbuh One via the broken recommissioning module is both heartfelt and heartbreaking, saying that while he might not remember what he did, he wants to go on his greatest mission: being a father to his son.
  Numbuh Zero: Hello, son. If you are watching this, then we've won. Congratulations. I bet you're wondering why the Recommissioning Module broke. Well, I broke it. I had a hunch you'd want to get Numbuh Zero back. Well...look, I'd love to stay and have lots more adventures with the super cool Numbuh One and his team, but... I'm just not a kid anymore. I'm an adult. And I need to complete the most important mission of my life — being a good father to my son. So, you're the keeper of the Book now, Nigel. That's right. Keep a stiff upper lip and make sure everyone gets to write their own crackerjack story.
• And then we see Numbuh Zero once again as Numbuh One's Bumbling Dad, with no memories of being a KND operative.
  Monty: I say, old bean! What are you up to? Making some more 2x4 technobob thingies?
  Numbuh One: (hugs him, who hugs him back) No, Dad. I was just playing with an old friend.
  • Zero and his wife do get recommissioned one last time, in the Grand Finale... to say goodbye to his son, as Numbuh One leaves for the Galactic Kids Next Door.

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