The Appearance of Strange Visitors

is episode 161 of the original Naruto anime.

Synopsis
Two outsiders, Mondai and Potcha, come to infiltrate the village and gather its secrets by disguising themselves as Might Guy and Rock Lee, while the real Guy and Lee are out on a mission. They are oblivious to the fact that their disguises are fooling nobody, except for Naruto. Tsunade, Sakura, and Tenten string them along by giving them outrageous missions, preventing them from learning anything. The real Lee and Guy return to kick their impostors off into the distance, and discover that Naruto never realized the truth.

Trivia

 * Sansho, Inari, and Tsunami make a cameo when the "Disguised Guy and Lee".