When Zim had broken into Dib's house, it had been one in the morning, so he really hadn't thought it necessary to bring a disguise. Dib already knew he was an alien—Dib-stink's sister already knew he was an alien—so what was the point, really?
Oh yes, Dib-beast had a father, too. Really should have remembered that one.
The sound of heavy footsteps came from the hall. Zim and Dib paused in their duel to the death—or at least to decapitation—at the sound.
Dib briefly considered bursting from the room and trying to convince his father to vivisect Zim, but—after Professor Membrane's blatant refusal to see through Zim's ridiculous disguise—his dad would probably accept the Irken's 'pink-eye' excuse without a second thought.
Why, why did he have to be the only sane person on Earth?
Silently lamenting as he put away his lightsaber, and threw Zim's ray-gun out the window—where had Zim gone off to, by the way?—Dib climbed into bed and feigned sleep.
A few seconds later, the door creaked open and his father entered the room, silently paced over to stand by his bed, and sighed.
Then he started to sing:
(To the tune of "My Boy" as sung by Richard Harris/Elvis Presley. Take your pick.)
Professor Membrane:
"You're still a child, I know.
So I'll just have to wait.
Don't ask me to explain;
it's already too late.
Well, your 'mother' is dead.
Who'd feed Gaz if I died?
All those years in the lab
and my organs are fried...
So now you're all I have, my clone.
You are my blood, my flesh, my bone.
And, when you die, you'll die so I can live, my clone.
I know, it's hard to understand:
to play this needed part.
But know I'll think of you
with each beat of your heart.
I've sent robots through time,
I've played God with the best,
bent the world to my will
it's too soon yet to rest.
So now you're all I have, my clone.
You are my blood, my flesh, my bone.
And, when you die, you'll die so I can live, my clone.
Sleep on. You haven't heard a word.
Perhaps it's just as well.
Why spoil your little dreams?
Why put you through that hell...
Life's a conspiracy
As, one day, you will know.
But now, you're just a child.
I'll stay here, and chart your growth.
Because you're all I have, my clone.
You are my blood, my flesh, my bone.
And, when you die, you'll die so I can live, my clone..."
Professor Membrane left the room after that, though Dib could still hear him warbling away as he made his way back to the lab.
A few seconds later and the closet creaked open, to reveal a disturbed Zim.
"What was that?" asked the invader, staring at the door as though it held the secrets to understanding all the bafflements of human culture.
Dib shrugged. "My dad," he said. "I think doing that makes him feel better."
Zim was still staring at the door.
"When I was three, I hacked into my dad's computer," continued Dib, seeing that the alien wasn't going to speak. "Found out that I'm not Dad's biological son: I'm his clone. Apparently, prolonged exposure to Super Toast isn't great for a guy's life expectancy, so..."
"Then... that human thinks Dib-head is his organ-bag!"
"A... what?"
The Irken's eyes grew almost misty with nostalgia. "Ah, human grub-child. Zim remembers when he was a smeet, and his superiors first tried to harvest his organs..."
"So... this is normal for your culture?"
"Why wouldn't it be? Did you really think our worlds were completely different? If Earth had nothing of Irk in it, it would not be worth conquering."
"Huh."
"Not to mention—hey! Quit trying such meaningless distraction ploys, human stink-pool! They shall not save you from the terrifying wrath of Zim!"
...and the duel continued until stalemate was reached, courtesy of interference by Gir, who had decided to bury them all in fish-tacos.
Where he got the fish-tacos is a story best left for another day.