The master of dirt, king of slime, Capitão Feio, with the help of an army of monsters, conquered the world in a sudden strike! He wants to pollute all the world and there's just one person who can stop him: Mônica! If she fails, she'll be turned into a dirty monster and the world will be polluted for ever!

Mônica no Castelo do Dragão (which translates "Mônica in the Dragon's Castle") is a modified version of Wonder Boy in Monster Land, with Wonder Boy's hero, Tom-Tom, replaced by Mônica, a famous Brazilian comic character. Mônica is this little, chubby, toothy and extremely strong girl, who beats up any of the neighborhood boys who dare to mess with her. She's always carrying her stuffed rabbit named Sansão, which is in fact her strongest weapon.

The gameplay is pretty much the same in Wonder Boy in Monster Land, with Mônica side scrolling through twelve locations. She can brandish Sansão against her enemies by pressing button 1. Through her way, Mônica must defeat monsters who will leave gold when defeated. With that money, Mônica will be able to buy new equipment (in proper stores), such as boots, shields, dresses and stuffed rabbits. By pressing button 2, Mônica can jump; jumping at certain places several times will reveal hidden items. The PAUSE button will display the statistics screen, with the amount of life and gold Mônica's carrying, as well as special items and equipment. Mônica will also usually find, at the end of each location, bosses who will drop lots of gold and a key to unlock the exit. The levels have a time limit, and Mônica needs to find hourglasses that will appear out of nowhere in order to replenish her hourglass, thus avoiding losing health. She can heal herself by buying juice or vitamin drinks at bars, or going into hospitals.

All the text in the game (dialog, items' names) is in Portuguese, except for the life and gold meters and the round end screens (which are still in English).