For a long time, humans have polluted the ocean, thoughtlessly dumping their waste and defiling the waters that countless sea creatures call home. The inhabitants of the sea have suffered under their toxic influence. Finally, one particular squid has reached her limit and vows to make humanity pay for its selfish actions.
Armed with all the formidable powers of a squid, including powerful hair-tentacles, the ability to spew ink, and even control bioluminescence at will, Ika Musume rises from the ocean depths to exact revenge on humanity! She surfaces at a certain Lemon Beach House, a restaurant run by the sisters Eiko and Chizuru Aizawa. Believing them to be an easy first step toward global conquest, she immediately declares war on them—only to discover that she is, quite literally, a fish out of water! To make matters worse, she damages a section of the beach house wall while showing off her squid superiority, and is consequently forced to work as a waitress to cover the repair costs. Stranded on land after a thorough defeat at the hands of the Aizawa sisters, Ika Musume has to put her plans for world domination on hold.
Despite these setbacks, Ika Musume soon finds herself surprisingly at home in her unexpected role as Lemon Beach House's newest employee. Whimsical and hilarious, Shinryaku! Ika Musume follows her new life on the surface as she makes precious memories and meets many new people. With her newfound friends, Ika Musume is ready to take the world by storm, one plate of squid ink spaghetti at a time!