For one day in a thousand years, a fairy godparent can be bad, and now it is Cosmo's turn. The problem is: Cosmo is horrible at being bad. If he fails to be bad, then he must return to Fairy Academy for training for 500 years and Timmy will never see him again. Is Genghis Khan able to teach him how to be bad?
Timmy is sleeping until Wanda wakes him up. He sees the time is midnight. Wanda puts him in a net.
Now Cosmo gets a turn to be bad and he grows a beard. However, Cosmo is "bad at being bad" for example he thinks bad means getting soap scum on his finger. Cosmo is failing at being bad and risks being sent back to the academy for 500 years of training. If that will occur, then Timmy will never see him again. Then Cosmo destroys Brussels sprouts, but that is good for Timmy. Jorgen comes to say that he has painful plans for Cosmo, when he will return to Fairy Academy.
Jorgen hopes that Cosmo will fail at being bad and hopes to have a chance to be in charge of Cosmo. (Jorgen probably wants to get even for the times that Jorgen was demoted due to trouble that Cosmo caused, as revealed in another episode).
Next, Cosmo tries to give a static shock to a cat, but the static only hurts Cosmo.
So Timmy wishes that "the baddest" person will teach Cosmo how to be bad. Wanda magically brings Genghis Khan and his soldiers. The first lesson of being bad is: not to say 'thank you". Six hours later he and Cosmo have a plan to conquer the world, but then Genghis's babysitter interrupts.
Timmy dials the phone to ask Vicky how to be bad. She suggests that being bad is destroying the Earth. Hence Cosmo changes the planet into a bomb. Yay! Mission accomplished! Cosmo was good at being bad... but now the whole world is in danger. Cosmo succeeded just in time, because it is now midnight again... so the bad day ends, hence the bomb is cancelled.
Cosmo offers a candy to show he is nice again, but Wanda now has a beard. Now it is Wanda's turn to be bad. The end.