'Sella,' an ordinary marine university student, finds herself reincarnated as an extra in a novel. In order to achieve her dream even in this fictional world, she begins to intervene in the original plot. The one thing she wants to avoid at all costs is getting entangled with the shadowy antagonist, a naval admiral. However, she discovers a secret about this admiral, 'Ellaim,' who reigns over the lawless seas: he is actually a legendary merman, a Siren. Sella, an aspiring free spirit of the sea, finds herself repeatedly entangled with the unexpectedly kind Ellaim. At the same time, Ellaim desperately needs Sella to control his overflowing magical energy and survive. Amidst a fate that rushes in like a wave, witness the cool, healing-filled ocean romance that the two create together!