"Viva Dojo Challenge! Youth is All About Passion!" (ビバ道場破り! 青春はバクハツだ, Biba Dōjōyaburi! Seishun wa Bakuhatsuda) is episode 193 of the original Naruto anime.
After returning from another mission, Naruto found a poster for a dōjō. The dōjō (which was poorly constructed), turned out to be owned by Lee. After initially believing Naruto came to challenge him and slamming him through a wall, Lee explained he built his dōjō in the hopes that skilled warriors of various disciplines would come to challenge him so he could improve his skill for combat. After narrowly escaping Lee's request to face him, Naruto bumped into Guy and told him about Lee's dōjō. Guy, who was so proud of his student's dedication to improve, decided to help Lee by disguising himself (albeit a very poor disguise) and challenging Lee. Unfortunately, Guy was called by Tsunade to go and help Team 10 who reported some trouble. Unbeknownst to him and Naruto, a thief named Nanafushi who came to Konoha to steal its secrets impersonated Guy as he left. Naruto, thinking he was really Guy, dragged him to Lee's dōjō. Not wanting to break character, Nanafushi challenged Lee and was quickly defeated.
Nanafushi managed to convince Naruto he was just exhausted and had Naruto fill in for him. Knowing he couldn't beat Lee in direct combat, Naruto relied on various shadow clone tactics. Before the fight could be finished, Nanafushi (who now disguised himself as Lee) was knocked out cold by the real Guy. After getting their facts straight on who was who, they brought Nanafushi to Tsunade, unaware of who he was. Immediately after Tsunade gave them orders to search for and capture Nanafushi, they handed him over to Tsunade. This was the fastest mission that Naruto has ever completed successfully.