The series revolves around the eponymous Muromi, a mermaid with a loud and cheery disposition and terror of the denizens of the sea in her (seemingly infinite) free time, and Takurō, a no-nonsense human boy whose hobby is fishing. Muromi and her entourage of mythical creatures attempt to befriend the boy and involve him in their misadventures, much to his annoyance.
This is an unashamedly silly show, with witty and irreverent banter and ridiculous situations. There isn't much in the way of a plot, but the eccentric characters hold their own. The fodder for abuse by Muromi-san and her friends include various mythologies, aquatic life, and even the prehistory of the earth.
The quality of the animation is not consistent at times, but it is often used to good effect in the depicting dramatic and surreal turns of events.
