- Operation: Z.E.R.O.
"Tell the Kids Next Door we miss them!"
- Numbuh One having a Heroic B.S.O.D. after he believes himself to be responsible for Grandfather's return.
- When, after getting to know the real Sector Z, they reverted back into their Delightful form.
- Or later on, when we see Numbuh Zero once again as Numbuh One's Bumbling Dad, with no memories of being a KND operative. "Just playing with an old friend, Dad."
- Father's failed 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 being forced to fight his former teammates. Fortunately, Numbuh Two's birthday suits help turn them back to normal.
- Numbuh Five's heroic sacrifice.
Numbuh Zero: "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.
- Numbuh Zero's final message to Numbuh One via the broken recommissioning module at the end of Operation Z.E.R.O. 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.
- Zero does get recommissioned one last time, in the series finale... to say goodbye to his son, as Numbuh One leaves for the Galactic Kids Next Door.
- In earlier episodes, the whole process of decommissioning is glossed over. Then in the opening of "Operation: M.A.U.R.I.C.E.", we're introduced to an operative, Numbuh Nine, who's a good friend of Sector V. It's his thirteenth birthday and he's willingly going to be decommissioned. The combination of his speech, his screams while getting his memories wiped and walking out as a sullen teenager with no memories is pretty damn depressing even if it was all an act. And the Crowning Music of Sadness makes things even more emotional.
Numbuh Five: My sister's a teen! The greatest Kids Next Door operative I ever knew is a teen! And just look at me, I'm practically a teen now!
- Decommissioning itself is a gloomy process. Imagine losing most of your memories from your childhood. Imagine forgetting your crushes and your best friends, and acting like they never existed.
- Numbuh Five's Growing Up Sucks speech from that episode.
- Vin Moosk's Heroic Sacrifice at the end of "Operation: K.N.O.T.":
Numbuh Two: Who was that guy?
Numbuh One: ... A comrade.
- "Operation: F.O.U.N.T.A.I.N.". It's pretty sad when they begin to contemplate what will happen to Leona when 300 years of suppressed aging will catch up to her. Even after they save her, and she double crosses them, and they save her again, as 1-4 year olds. Waving goodbye to her fountain, watching it crumble to the ground. Her finding a different one and laughing made up for it, a little.
Baby!Numbah Four: I thought we came here to save Weona.Baby!Numbah Five: Change of plans! She's an adult and a jerk!
- Also that near the end it seemed Leona was going to pass away from old age
- Operation K.A.S.T.L.E.: King Sandy torturing Rainbow Monkeys...and giving a Scarpia Ultimatum where he'll dip Rainbow Monkeys in nacho sauce if she doesn't agree to marry him.
- "Operation: S.L.U.M.B.E.R.", where Numbuh 86 has to invent a super-secret, girls-only mission to get people to come to her slumber party.
Numbuh 86: I know it's unusual, but my mom is forcing me to throw a slumber party and... I had to invite someone.
Numbuh Three: Why didn't you invite some of your own friends?
(Numbuh 86 hangs her head)
Numbuh Three: Oh.
- It makes the moment at the end, where Numbuh Three insists that she's 86's friend, all the more heartwarming.
- Lizzie's breakup with Nigel is unexpectedly serious. She recognizes that Nigel's duty to the Kids Next Door will never give her the boyfriend she wants and decides that breaking it off would be for the best. Might not drive someone to tears but it is a well-handled moment of drama that can inspire some empathy from the viewer.
- "Operation: M.A.T.A.D.O.R.": Numbuh Four is having fun in his time as a "bully fighter", playing matador to caffeinated adults who act like bulls. The heavy stuff comes when at the championship, his next adult to fight is his father (whose face is shown for the first time). Also, when he learns the dark side to the fights: adults are lured, captured and thrown into pens and force-fed enough caffeine to make them charge at anything, even their own kids. Numbuh Four's Dad got roped into the whole mess because he believed he was going to a seminar... on being a better dad. OUCH.
- In Operation C.A.R.A.M.E.L, we learn why Heinrich holds ill will towards Numbuh 5. 5 years, 5 months, and 5 days earlier an incident happened that gave Heinrich delicious caramel unknowingly at the cost of what he valued most. Turns out he used to be a female and she had lost what she had valued most, her beauty. Assuming Heinrich is 10 like Numbuh 5, this means Heinrich lost what he had valued most when he was only 5. At such a young age losing something that important to you would do severe psychological damage.
- Anyone else feel bad for the Chief Safety Bot in Operation S.A.F.E.T.Y.? Well-Intentioned Extremist, yes, but it only wanted to make a world safe for kids. When it realized it had in fact hurt a child (thus paradoxically violating its programming), it realized since it was in fact a threat it must neutralize itself. After spiraling out of control, it pauses and says only "please get home safely" before it wraps everyone in bubble wrap and proceeds to blow up the space station, destroying itself and sending everyone back to earth unharmed.
- Right before that, we have Numbuh 4 freaking out and crying upon seeing his baby brother Joey injured. Joey actually faked it using ketchup, but it can still strike a chord for some.
- "Operation T.R.E.A.T.Y." has Chad and Nigel's mentor and pupil relationship. After seeing the sweet flashback at the beginning, seeing Nigel's forlorn expression as he remembers and how the two attempt to kill each other episodes makes one want to weep for their deteriorated relationship.
- "Operation: O.O.M.P.P.A.H" Numbuh 1's father takes him on a fishing trip to bond. After a day of boredom and annoyance, Numbuh 1 snaps at his father and the camera pans up show his face (notable, as prior to this, it had been obscured, so showing it swiftly shifts the mood) and he just looks heartbroken.
Monty Uno: I'm sorry, Nigel. I just wanted to be your friend.
- Then there's Nigel's reaction to his father's response. You can tell that he realized how horrible and ungrateful he must have sounded. It gets even worse when his father is kidnapped and he never got a chance to apologize. Just imagine the guilt he was feeling.
- The ending of "Operation: I.N.T.E.R.V.I.E.W.S." Adding extra weight to it? It's the audience's goodbye as well. It's been a great six season run, and seeing it come to an end is just heartbreaking.
Numbuh One: Just promise me one thing — that you'll never grow up! Even if you're a hundred-years-old and don't remember a thing about our times together, you'll still be a kid at heart, okay?
- The last image◊ from the series is enough to make anyone bawl.
- In a blog post, Tom Warburton describes how he eventually came to write the farewells scene, and basically wrote himself to tears, saying goodbye to the Kids Next Door like they were his own children.
- The recent teaser is full of this and Nightmare Fuel: Numbuh 1 is about to unleash something on Earth, which he says is infested with adults while Chad pleads with him not to, protesting that their families and friends are down there.
- After the arguments between the various aliens is silenced by whomever is in charge, they say its his choice to make. After a moment of hesitation, he makes a salute while stoically shedding a single tear and proceeding to activate the device.
- In a meta example, the ultimate reveal that despite global fan response and even some positive prospects among CN executives, G:KND failed to be greenlit for production.
- Surprisingly, "Operation: A.F.L.O.A.T." manages to become a tad upsetting as it takes a turn towards Jaws-level territory as Sector V winds up being picked off one by one (except Number 4, who drowned in the asparagus sea), with it being played as seriously as possible given that the Jaws equivalent is a great white asparagus monster. And this is despite Stickybeard saving the day in the end.
Tear Jerker / Codename: Kids Next Door
Codename: Kids Next Door may be a kids show, but it can still be sad!