Zenmai is a shining example of good in his village, but the wind key in his head is big weakness. If he is struck by lightning, the polarity of goodness inverts and he becomes the evil Akutare Zamurai. Under this persona, he plays pranks on and makes life difficult for everyone. The dumplings on his Dango-ken also change to slimy balls which turn others evil when eaten.  (Wikipedia)

200 years ago, Zenmai Zamurai was a thief by profession. While stealing from a house one night, he fell down a well and died. He was then resurrected by Daifukunokami and a wind-up key was placed on his head. If the key wound down, Zennosuke would die. The only way the key can be wound is by Zennosuke doing good deeds. In order to achieve this end, he is given the Dango-ken, which is a sword with Japanese Dumplings skewered onto it. Whenever Zenmai Zamurai sees people doing injustice or generally being out of control, he uses his sword to shoot the dumplings into their mouths. When the dumplings are eaten, the people suddenly become happy and realize the error of their ways. He helps out the people of his village in many ways, big and small. Whenever he has done well, a rainbow comes out of the sky to wind up his key. 
 
(Source: Wikipedia) 
 
Note: From the third season onward, the episodes were broadcasted two at a time.