In Mari Land, a picturesque world entirely populated by Ridiculously Cute Critters, two miscreants named Kuromi and Baku are imprisoned in a dungeon. One day, they obtain the Melody Key, a magical instrument with dark powers, and escape from the prison. They head to the human world in order to collect Black Notes by making people's dreams come true in horrible ways, with the eventual goal of resurrecting the Spirit of the Dark Power. Meanwhile, an acquaintance of Kuromi named My Melody is suspected of helping the two fugitives escape. She is ordered to go to the human world to arrest them and save people's dreams. My Melody arrives in Yumemigaoka City and helps a junior high school student named Uta Yumeno when she falls victim to Kuromi's Melody Key. The two become friends, and My Melody starts living with Uta. After that, My Melody and Uta are involved in various troubles caused by Kuromi, but they solve them using My Melody's Melody Takt, a magical conductor's baton.As each season of the series progresses, newer tasks fall to My Melody and her friends. In ~Kuru Kuru Shuffle!~, My Melody must collect Pink Notes while Kuromi must collect the Black Notes so they can fulfill their wishes. In Sukkiri♪, both My Melody and Kuromi are candidates to become the new Princess of Mary Land. Kirara★ however is a prequel story, taking place before the events of the three anime series. This series revolves around Kirara collecting the missing pieces of the Wishing Star so that she can return to the human world.The irreverent, genre-deconstructing humor saves the show from tasting like diabetes.
And Now For Something Completely Different — Sukkuri in general, which distills the formula of the previous series so that every episode is half-length. While the formula is similar, there are many inherent differences, such as how Kuromi doesn't actually use nightmare magic on other people this time.
Cute Witch — My Melody, with emphasis on the cute.
Dangerously Genre Savvy — When Hedgehog-kun, who is shown to be the one that restores normalcy after My Melo's battles, tries to roll back after Hiiragi starts playing the Song of the Dark Power, DAR-chan zaps him. He doesn't recover until after the world's been saved.
DAR-chan gets even more savvy the second time around.
Demonic Possession — The Big Bad, Spirit of the Dark Power, has no physical form, and must possess other beings to further its goal.
Disproportionate Retribution — Arguably the root cause of the entire series. My Melo has apparently been accidentally embarrassing Kuromi for years, and messing up her life. She has done this literally thousands of times, and most of them are the result of My Melo trying to help Kuromi.
The Ditz — My Melody, and some inhabitants of Mari Land, in particular the elephant King.
Hostile Show Takeover — Referenced when My Melo is about to be cooked by the Monster of the Week, and Kuromi exults that "Starting next week, 'Pretty Lil' Devil Girl Kuromi-chan' will air in this timeslot!"
How Do I Shot Web? — Kuromi loses the instruction manual for the Melody Key, and has to figure out how to work it.
Refusal of the Call — This is Keiichi Hiiragi's whole shtick in the second series, being forced to become a super hero as penance for his actions in the first.
Revenge Before Reason — Kuromi is motivated by the imagined slights and grievances, often very minor, she has suffered thanks to My Melody - she keeps a thick notebook on these. Given Melody is such a clueless Ditz, most of the grievances are better off laughed away.
Sibling Rivalry — among Keiichi and Jun Hiiragi, and among their respective valets.
Sickeningly Sweethearts — one downright creepy, unnamed pair appear occasionally in the series. They always fall victim to Kuromi's mischief.
Tarot Motifs — The cards in the Melody Compact in Shuffle feature the summonable residents of Mari Land in different tarot poses. Notable is Rhythm as "The Fool," the King as "The Hanged Man," and My Melo herself as "The Magician." A few are even minor arcana - Bako is the "Four of Swords" and Santa Claus the "Ace of Pentacles."
Yandere — Kuromi, for both Keiichi and quite possibly My Melody. In almost every episode, she says she will gather Black Notes for Keiichi, something that involves turning the dreams of the depressed, the hopeful, and the occasional kindergartner into horrific nightmares, and at the end of season 1 seems quite willing to let the world be destroyed if it means Keiichi will like her. The only reason she joins up with My Melo is to save him. As for My Melo, she apparently wanted to be friends with her when they where little, then made it her life's mission to destroy her. The judge of Mari Land even states that her obsession is one of the main reasons she's being locked up.