"They fear daylight, multiply underwater and turn ugly when fed after midnight..."
A clumsy engineer stumbles upon a discovery while visiting a Chinese antique store. He purchases a mogwai, a rare animal of which most people ignore the existence. He offers it to his son, for Christmas. Then, he explains the strict rules that need to be followed to take care of such a special creature. First, the mogwai must never be exposed to light. Then, it must never be fed after midnight. Finally, its hair must never be in contact with water.
By accident, light strikes the mogwai. However, nothing really bad happens. On the other hand, when it touched water, a few fur balls flew off of its back and grew to become other mogwais. These newborns are not as cute and respectful as their parent. They spit, make noise and play tricks.
One day, the worst happens: they are fed after midnight. Immediately, they lock themselves up in their cocoons where they spend a few days until their metamorphosis has ended. Then, they flee into the street where they wreck havoc and murder just for the fun of it.