The time when Hattori fell sick (Hattori's faked illness) (仮病はつかれるでござるの巻) is episode 625 of the Ninja Hattori-kun 1981 anime.In a bid to get a comic from Yumeko, Kenichi pretends that Hattori has a broken leg - so Hattori has a lot of work to do!
At lunchtime, Mrs Mitsuba calls her children to the kitchen. Shinzo and Kenichi rush in, and Hattori hops after them. While Shinzo and Kenichi gobble their food loudly, Hattori eats standing with one leg on his chair. When Mrs Mitsuba reproaches them, Kenichi says that Hattori is using the ninja technique of multitasking while standing on one leg. Hattori, embarrassed, modestly asks Mrs Mitsuba if he can do it.
Next day, at Kenichi's school, Yumeko brings a comic with her. Kenichi asks if he can borrow it, but Kemumaki comes to ask for it too, and the boys nearly get into a fistfight. In a bid to break up the fight, Yumeko suggests they play Rock Paper Scissors. After Kemumaki uses some chewing gum to turn Kenichi's hand into a rock, the boys have their round and Kemumaki, having cheated unbeknownst to Yumeko, wins the game. Angry at Kemumaki, Kenichi lies to Yumeko that Hattori broke his leg while training, and Yumeko gives Kenichi the comic, much to Kemumaki's displeasure. Yumeko also decides to visit Hattori while he's "unwell", which gives nasty Kemumaki an idea...
At home, Hattori incredulously asks Kenichi if he really lied to Yumeko. Horrified, he knows that he has to act as though he has broken his leg, but he complies for Kenichi's sake.
Later on, Hattori, wearing a cast on his right leg, lies on Kenichi's bed as he begs Kenichi to let him have a look at the book too. All of a sudden, Yumeko and Kemumaki turn up with gifts: Yumeko brought flowers and Kemumaki brought a skateboard, though Yumeko points out that Hattori can't ride it because of his "broken" leg. Mrs Mitsuba turns up and serves tea for Kenichi, Yumeko and Kemumaki. Kemumaki, "helping" Mrs Mitsuba, puts the tea tray onto the coffee table. In the process, he deliberately sends the skateboard crashing into Hattori's leg - really hurting him this time. He then tries to give him a Koga medicinal drug, but misplaces it. Mrs Mitsuba then leaves them to it, and Hattori cooks up a good plan. Using his "injured" leg, he kicks Kemumaki in the back (twice). Kemumaki shifts spots, but Hattori kicks him again, this time hitting him in the back of his head (ouch!). He tries one last time, but he mis-aims and kicks Yumeko! Yumeko immediately suspects Kemumaki. He tries to reason with her - but she slaps him and storms away. Determined to put a stop to this, Kemumaki runs after her. Hattori, cooking up a plan, gets on his skateboard, rides on his hands, and chases after Kemumaki. He then tauntingly thanks Kemumaki for the skateboard. Kemumaki shouts at Hattori for making Yumeko leave, then continues to follow her. Hattori leaves for home - but on the way, he crashes into a lamppost and bumps his head, concluding that even skilled ninjas need to be careful.