Disclaimer: Yeah, I still don't own it, sorry.
Because Voldemort needed to lighten up.
James filled the bucket with goop (a weird concoction that had resulted from his futile attempts at cooking) and placed it over the doorway with a flourish of his wand. When Sirius arrived later this evening he would get a surprise. Dear old Padfoot would find himself covered in immobilizing goop. James grinned triumphantly at the bucket, which Lily, walking by with Harry to the couch, could not possibly fail to notice.
"James," she said suspiciously. "What is that?"
"Oh, just a little surprise for my very best friend in the entire world!" James replied cheerfully.
"Huh. Well, if that surprise stains the carpet..." she let the sentence trail off threateningly as she fixed James with a stern glare. James smiled sheepishly and pointed his wand at the carpet. "Oh no, love. Impervius! Perfectly stain-free."
Harry giggled. "Harry!" James cried. "My favorite little man! D'you like bubbles? Bubbles, Harry?" As he spoke his wand spouted out a fountain of bubbles. Delighted, Harry clapped. "Pongs bubble!" Lily laughed and made her wand produce bubbles, too as the small family sat on their couch. Harry very much enjoyed bubbles and how they popped when he touched them.
Beaming, James briefly looked out the window to see a flash of robes. His smile became a mischievous smirk. "That's right, Padfoot, walk right in... to your DOOM!" Harry looked up. "Pa'foot?" he asked, excited by the mention of his godfather.
"'Doom', James?" Lily repeated, looking both exasperated and amused. But she made no move to warn Sirius; she hadn't entirely forgiven him for eating all of her Chocolate Frogs a few weeks ago.
The door swung open quickly, and the Potters had but a second to register that that was definitely not Sirius before the bucket upended the goop upon their unwelcome visitor and Voldemort found himself completely immobilized.
For a second Lily and James were frozen with shock, but then their wands were pointed toward the frozen glob. "STUPEFY!" They cried simultaneously, and the goop was blasted off to reveal Voldemort, unconscious.
"Lily, get to the Auror office quickly and take Harry with you, I'm watching him." James said grimly. Harry had noticed the change in atmosphere and was staring solemnly form his parents to Voldemort. "Pa'foot?"
Lily had scooped up Harry in one arm and walked out the door, Apparating the second she reached the boundaries of their protective charms. James had cast a Patronus charm and was speaking to it. "Sirius, are you okay? Voldemort just walked in on us, it was Peter, Peter was the spy, and I'm probably only alive just now because of a prank for you-- it would've been good, by the way-- and I've got him out cold here." The Patronus bounded off out the window, only to circle back as Sirius appeared outside his door with a loud crack.
"James! James, are you-- Harry and Lily-- it was Peter, and I suggested him-- Voldemort!" And he too leveled his wand at Voldemort, pale and shaking.
"Everyone's fine Padfoot, thanks to a prank I had set up for you. Lily went to the Ministry with Harry, they're safe." James smiled weakly at his best friend. "Happy Halloween, by the way."
There was a series of loud cracks like ice breaking and what appeared to be the entire Auror office rushed into the Potter house.
Mad-Eye was standing beside James, who said, "Send people to find Peter Pettigrew, he's been spying for Voldemort." Mad-Eye growled assent and barked orders. A small chunk of the Aurors walked out the door and turned on spot. They were gone with a crack.
"You too, Prongs."
"Pongs! Pa'foot!"
Cat Out