Kemumaki disguises as Hattori (Mask effect) (いたずらは自分の顔での巻) is episode 560 of the 1981 Ninja Hattori-kun anime.
On seeing Kagechiyo disguise himself as Shishimaru, Kemumaki goes undercover as Hattori and causes trouble. Even though everyone is unaware of everything at first, Yumeko discovers that Hattori has an evil twin. Will Hattori be able to clear his name before it's too late?
Kemumaki is pretending to be a goody - goody two shoes as usual, helping Miss Aiko carry a large stack of books and taking off his shirt so that Yumeko can cross a muddy puddle. Back at his flats, Kemumaki is washing his shirt, just as Shishimaru arrives. He opens the fridge and takes a piece of fish, but then gets told off for revealing his identity. While Kagechiyo removes his disguise, Kemumaki gets an idea: to disguise as Hattori and get upto some naughty business...
Wicked plot number one: Tsubame is delivering groceries to Mitsuba house, when Kemumaki, as Hattori, comes rushing towards her and shredding her robes, thus revealing her knickers. Tsubame declares her hatred for "Hattori" and runs off in tears. Kemumaki takes his Hattori mask off and sniggers loudly at the situation.
Wicked plot number two: Mrs Mitsuba is sorting out the laundry, when "Hattori" covers a bedsheet in black paint. He then blows a raspberry at a scandalised Mrs Mitsuba, before flicking some paint at her.
Meanwhile, the real Hattori returns from training, only to face a livid Kenichi, a furious Shinzo, an irate Shishimaru, an enraged Mrs Mitsuba, and a fuming Tsubame (who has repaired her robes). Hattori tries to sort things out, only to be brutally thrown out of an upstairs window by Kenichi and Mrs Mitsuba. Tsubame then shouts that she doesn't want to see his face again. Undaunted, Hattori then tries to think about who could be behind these outrageous deeds, and (no guesses there) suspects Kemumaki.
Wicked plot number four: Yumeko is returning from her music class, when Kemumaki, still in the guise of his rival, uses a fishing rod to reveal her knickers too. Yumeko then expresses her fury at "Hattori" when the real Hattori then comes.
The real Hattori and the fake Hattori then engage in a fierce duel. The real Hattori then places another mask on the fake Hattori, who runs off. Yumeko then recovers the Hattori mask from the duel and shows it to Kenichi, Tsubame, Shinzo and Shishimaru.
Meanwhile, at the Shinto temple, Kemumaki sobs to himself that he has been exposed. Hattori then turns up and assures him that his identity hasn't been revealed, but the mask has been attached to his face with some extra super sticky liquid glue, which will only wear off after 12 hours. The rest of the gang turn up and chase after the impostor, and Hattori says that whatever happens, your true identity will be revealed if you try to pose as somebody.