SpongeBob sees a seahorse, tames it and calls it a Mystery. Spongebob brings a Riddle to Krusty Krab, but she eats all the food. Mr. Krabs says SpongeBob has to get rid of the horse, and he does it anyway. In the end, they discover that the Mystery ate Mr. Krabs' money, thinking it was food.
P.S. Patrick is incomparable.