Cosmi is vulnerable in his small alien form and must first beam up a nearby earthling in order to create a disguise for himself. Once he has beamed a person up, he can change his form by stepping under a UFO. Each earthling has a different ability that can be used for causing mischief, such as boomboxes, vacuum cleaners or bazookas. Successfully knocking over earthlings yields coins that are required to complete the goals set in each level. Once pranked, the earthlings will become hostile towards Cosmi and try to attack him.
While disguised, Cosmi can take two hits, the first leaving him in underwear (a possible homage to the Ghosts 'n Goblins series), while the second one reverts him to his vulnerable alien form. Earthlings will continue to chase and attack Cosmi, forcing him to lose coins, until he can find another UFO to change into another disguise. At certain points in the level, a Panic Time will occur where something crazy in the stage happens for a short amount of time.
- Cosmi - The son of Cosmi. Sr, he considers himself to be a pro in Mischief, even though he's clumsy and arrogant. He can transform himself into Super-Cosmi by using a special item. He thinks that Master Itazura shouldn't worry about him.
- Becky - The self-proclaimed friend of Cosmi. Cosmi thinks of her more as a nuisance than a friend, considering her obsession to be always with him (and pulling pranks on him). She looks similar to Cosmi, but with white skin, blonde jelly-like hair with two snail-like eyes on top (which might mean that her hair is alive as well), a red dress, red boots, a red lady hat and white pants under her dress. She can teleport with a special item. In the end, she becomes egotistical and demands that everyone should love her.
- Mumphy - He is quite the lovely baby of his family, and hasn't left his planet until now. His nickname is the "Ball of Fire", which fits his appearance well. He is a robe-clad ball with eyes and a mouth, with pink skin color and a feather on top. His special item causes Volcanic Eruptions. In the end, his mother reveals that he is adopted, and shows him a picture of his real mother.
- Baron Von Proptop - A vagabond of all trades, he travels from world to world to seek new life and new civilisations, but collapses from exhaustion upon reaching Earth. He is saved by a friendly nun, and taken care of by her. He finds out that they suffer from an economical crisis, so Proptop decides to thank the nun by winning the contest and giving the prize money to them. He wears a tall French-polis hat with a propeller on top, a red scarf, goggles, and a green jumpsuit with large sleeves. His special item causes everyone to tie themselves up and drop coins. In the end, he wins and returns to the nuns, leaving the money to them.
- Princess Cleo - A princess from her own world. She has never had a boyfriend and enters all kinds of competitions to raise her fame. She decides to enter the Michief tests to try and raise her fame again. She looks like a mermaid with pink fish skin on her tail, pink hair, a very oddly shaped crown, a white trenchcoat and a human face. She can summon a UFO whenever she wants. In the end, she finds true happiness from witnessing Earth's vast oceans, and decides to spend the rest of her life in the sea.
- Magnetron - A genius in mechanics. He creates a large machine, which he operates called "Magnetron" for the contest. He himself is a small green blob with only one eye, a cat-like mouth and a shell on top of his head. His robot has horns, goggles, a large clunky blue body, magnets for hands and small legs. He can use a special item to drag in every coin for a limited amount of time. In the end, his success leads to the mass-production of his robots, and he becomes extremely wealthy.
On Planet Mischief, there is a tradition that once someone turns 3 years old, he/she must travel to a planet and perform "Mischief" and cause widespread panic. Cosmi, a baby alien of that age, travels to Earth for his mission, in order to impress his father, the Master of Mischief, Cosmi Sr.. Earth is considered by them to be the greatest challenge, since humans apparently go hopping mad over anything extraterrestrial. Sadly for Cosmi, he crashes into a TV-satellite from a town called Coco Town.
When crashlanding, he manages to hide in an alley in the town, where he is saved by Earth's Mischief Master, Master Itazura. He takes him to his Dojo to train, and lets Cosmi move on with his mission, on 8 different locations: Coco Town, High Stakes Hill, Pranksylvania, Pharaoh Island, Big Booty Bay, Frontiersville, Raccoon City and finally Cosmopolis. It is revealed that Cosmopolis is a trap for "Anti-Alien Forces" to capture all the aliens on Earth by disguising themselves as aliens. Itazura uses their trap as the final test for Cosmi, and challenges him personally there. Cosmi wins and travels back to Planet Mischief to celebrate, but crashes into another TV-station, landing on Earth again. This time, he lands in the middle of Coco Town Square. Of course, panic arises and reporters try to get a snapshot of the little alien, but Cosmi accidentally pushes someone's camera around, causing it to take a photo of the crowd, thus revealing that several of them are also aliens. Lastly, all of them are seen escaping.