(You don't actually get to the punch line of this story in this chapter, believe it or not. But hopefully hang in there, I think it's going to be worth the wait, maybe...)
"But why me?" Iruka asked for the dozenth time since the Hokage had given them their latest assignment.
Kakashi grinned and held out the dress for him. "Stop complaining and put it on."
"Nobody would believe me in drag."
The horrible thing was, it was the truth. Iruka, presented with the parameters of the mission, had dutifully followed the Hokage's instructions and paid attention to studying Naruto's sexy-no-jutsu, despite the repeated loud guffaws from his former student. Kakashi hadn't said a word, and hadn't bothered to try.
Iruka had assumed that, like any serious shinobi, he was applying his formidable Sharingan duplication technique and therefore didn't need the practice.
He learned otherwise when the Hokage asked to see the fruits of their study. Iruka had, despite considerable embarrassment, managed to produce a fairly plausible-looking "Iruko" version of himself.
Kakashi had just smirked and turned himself into a silver-haired old auntie with missing teeth and a squint in one eye and a horribly lascivious leer.
Kakashi had been paying attention, Iruka had decided dismally. But, this being Kakashi, he had been paying attention to the lesson that Iruka had overlooked -- namely, that the one of them who produced the more normal-looking sexy-no-jutsu form would be the one who spent the year of their undercover assignment in drag.
So it was nothing but the pure, unadulterated truth when Kakashi said with perfect self-justification, "I'm terrible at Naruto-kun's sexy-no-jutsu."
"So am I!" Iruka protested.
Kakashi's grin was lecherous even through the mask. "I beg to differ there. You don't need sexy-no-jutsu."
Feeling an all too familiar blush in his cheeks -- how did the scarecrow manage to do things like this to him? -- Iruka said, "Neither do you, you know!"
"But I'm not the one going in drag."
"But why me?" Iruka wailed again. "If you'd wanted to, I know you could have used Sharingan on him and learned it better than I could..."
"Ah, but there's the key," Kakashi said. "I'm quite happy not knowing, thank you."
"But the assignment..."
"...Only needs one of us in drag. Lucky for me, you studied quite admirably."
"Put on your dress." Kakashi was deriving far too much entertainment from this entire fiasco, Iruka decided.
There was a knock on the door, and the Hokage called, "Is everything all right?"
"Just fine," Kakashi called back lazily, despite Iruka's furious glare. "We'll be there in plenty of time."
"By Iruka-sensei's definition of 'on time', or by yours?" was the rather skeptical reply.
Kakashi just laughed, and suddenly both of his hands sprouted shinai. "By his definition," he replied, and advanced on Iruka with an entirely too manic gleam in the one eye that was visible. "Now, are you getting undressed by yourself, or am I going to provide assistance?"
Less than three minutes later, Kakashi escorted a fiercely blushing and far more femininely-endowed "Iruko" out of the house, wearing an ankle-length blue dress cut up to the thigh and showing far more cleavage than (s)he was comfortable with.
Tugging again at the neckline of the dress, Iruka growled, "Why did you have to let Sakura pick out my clothes?"
"Because she's got much more modest taste than I do," Kakashi said, grinning again. "Although, if you're too unhappy with that one, there was this centerfold in last month's Icha Icha Paradise who was wearing nothing but a gold collar, a leash, and a..."
"This is fine," Iruka said hastily, tugging the collar even higher. "This is perfect. Wonderful. Remind me to compliment Sakura-kun on her excellent taste."
The Hokage and the infamous trio of Group Seven were waiting for them at the edge of the village, of course. Sasuke was silently holding two horses and looking bored. Sakura's eyes lit up when she saw "Iruko," and she drove her elbow into Naruto's ribs.
"I told you I had good taste! Iruka-sensei, you look gorgeous!"
"Sakura-kun, that's not helping," Iruka said, feeling his face burning yet again.
"Oh, but you've got to do something better than that with your hair," the girl told him sternly. "You can't just drag it all on the top of your head and tie it with a ratty old shoelace anymore! Get down here so I can fix that--"
Kakashi, meanwhile, was leaning on the trunk of a nearby tree, too doubled over with hilarity to speak. Rolling his eyes, Iruka obediently knelt so that Sakura could "repair" his hair.
When she reached into her bag and pulled out a flowery blue hairpin dripping ribbons and pearls, his nerve started to give out. "Sakura-kun..."
"It's for the good of the mission, remember," she reminded him, with a decidedly mischievous quirk to her grin. "I'll send you care packages. You'll probably need them. You can't wear a blue hairpin with that forest-green dress, after all, and Kakashi-sensei is under strict orders not to let you out of the house with a shoelace in your hair!"
The thought of shaving his head bald was starting to hold some appeal. "...Yes, ma'am," he said in weary resignation.
Naruto had been entirely too silent through all of this. Dreading the number of possible causes, from incipient hilarity to not-quite-sprung-yet collateral-damage-inducing mischief, Iruka looked up at the boy...
...who was staring down "Iruko's" cleavage. And drooling. With a trail of blood trickling out of his nose.
"You little pervert!"
Since the little brat was still too busy gawking and drooling to dodge, Iruka's fist connected quite solidly with Naruto's head; the boy went at least twenty feet straight up, and there was a crash and rattle and several birds flew squawking out of the tree Kakashi was leaning against laughing his brains out.
Then there were some more rustles, and Naruto's head peeked back out of the tree, with a fist-shaped red mark on his face, an ear-to-ear grin, and a jutsu-induced pair of binoculars. Which focused on the predictable spot.
Shaking all over, Iruka said to Kakashi, "This is your fault, you know!" He dragged a blanket out of a saddlebag, wrapped it around his neck and shoulders, and climbed onto his horse.
...Forgetting that he was wearing a dress. One slit up to the thighs.
Sasuke pitched over in a dead faint.
The Hokage turned away hastily, a handkerchief shoved against his face.
Sakura turned a series of interesting colors ending in sort of pinkish-purple, and said, "Iruka-sensei, wait here just a minute, I think we need to get you some more things. Leggings, for example. Leggings to wear under the dresses. Ankle-length. --White, black, and gray should go with anything. You can't possibly not color-coordinate those. Yeah. That sounds... yeah..." And she headed off at a dead sprint.
Kakashi, meanwhile, was still trying to break a rib laughing.
"I hate you," Iruka said, and turned his horse around so that at least he wouldn't have to watch the shambles his life was starting to become as he waited for Sakura-kun to get back from her emergency shopping expedition.