"Monster Strike: Mermaid Rhapsody" is a fantasy-adventure anime that centers on a young mermaid princess named Nanami, who embarks on a journey to save her underwater kingdom from impending doom. The story begins with the peaceful underwater realm of Mizugami facing a catastrophic threat as the Ocean Crystal, the source of the kingdom's power and balance, begins to lose its energy. Without the crystal's protection, the kingdom risks destruction, and its inhabitants face an uncertain future.

Nanami, determined to restore the Ocean Crystal, ventures to the surface world in search of a legendary artifact known as the Mermaid's Tear. This artifact is said to possess the power to rejuvenate the crystal and save her home. Along the way, she encounters a group of Monster Strike players—Ren, Aoi, and Yuki—who become her allies. These surface dwellers, skilled in the game of Monster Strike, agree to assist Nanami in her quest, using their strategic abilities and monster companions to overcome challenges.

As the group travels across diverse landscapes, they face numerous adversaries, including rival factions and powerful monsters, all vying for control of the Mermaid's Tear. The narrative explores themes of friendship, courage, and the clash between the surface and underwater worlds. Nanami's growth as a leader and her deepening bond with her human companions form a central emotional arc, while the stakes escalate as the Ocean Crystal's deterioration accelerates.

The series balances action-packed battles with moments of character development, highlighting the unique abilities of each team member and their monsters. The quest for the Mermaid's Tear culminates in a climactic confrontation that tests the group's resolve and unity. Through their combined efforts, Nanami and her allies strive to restore balance to Mizugami, ensuring the survival of her people and the harmony between worlds. The story concludes with a resolution that underscores the importance of cooperation and the enduring connection between humans and the mystical realm of the sea.