It started with Mizuki's "extra credit" project, with the objective being to infiltrate the Hokage tower and steal one of the most valuable scrolls in Konoha. With a decade of pranking experience, Naruto knew that he needed to scout out the area before committing to anything, and he had just the trick. His henge wasn't an illusion, it was physical, solid-real. Therefore, when he kidnapped the receptionist it was with the intention of extracting every bit of information he could, in order for him to perfect his cover.
Naruto had absolutely no idea how to find out everything he needed to from the asshole who worked at the front desk, but apparently the guy really didn't like being in close proximity with the demon-brat-he spilled his guts out almost immediately. Problem solved, Naruto slipped into the persona of Tetsuo.
Tetsuo was approximately twenty five, and a retired chuunin. He had chosen to pursue a career in administration after a mission-gone-wrong resulted in a debilitating knee injury, and he was admittedly the best there was when it came to organisation. He walked in such a way as to partially disguise his limp, although he had long since forgotten most of his shinobi skills. His voice was high, and his articulation irritatingly precise, although when panicked he spoke with the low-born civilian accent. Although his primary duty was to monitor those who requested entrance to the tower, he was allowed to access every room, including the library. His arrogance was legendary, and he wore the most expensive clothes the market had to offer, including a purple shirt that had cost a month's wages, and trousers dyed so deeply black that only five pairs had made it to the storefront.
As Tetsuo Naruto discovered that the ANBU changed shifts every four hours, and though the transition was instantaneous, towards the end of their shift the guards would be just a little bit laxer. The chuunin who sorted out the excess paper work took a coffee break that coincided with the last 20 minutes of ANBU Guard four's shift, and within that timeframe floor four, just below the library, was completely deserted.
It was so much easier than the academy test. Poor Tetsuo couldn't quite forgive himself for betraying the Hokage, but Sarutobi didn't have the heart to tell Naruto exactly why a new receptionist was found that same week. He did, however, hand over a number of files to Morino Ibiki, and slipped into a folder was Naruto's report on the successful infiltration.