Once upon a time, there is a lion who likes to sleep.
"Jingle bell Jingle bell, ho ho ho merry xmas" mouse is disturbing the lion from sleeping.
The lion is very angry and want to eat the mouse. The mouse beg for life, and will repay the lion if leave him alive. So the lion releases the mouse.
One day, the lion being caught by hunter's net.
The mouse bites the net and save the lion.