As a low-ranking soldier in the mermaid army, you are suddenly thrown into prominence after an unexpected shark attack. Soon, the generals want you to help with the next stage of their war plans: infiltrating the human world. Your spy mission quickly falls apart, when life on land is unlike anything you would have expected: Humans have amazing technology, wonderful foods, and, much to your surprise, human males are not kept as slaves. As a literal fish out of water, it's almost too much for you to handle. Will you help your fellow mermaids conquer the humans? Or will you find love and decide to live on land...forever?
I only had a few ideas for this one. It could take place in the early 1900's, when submarines were coming into prominence. This is what prompts the undersea worries about people on land.
I was thinking one chapter introduction to the main character and mermaid land, ending with the shark attack. There is a challenge to survive, and the mermaid rises to prominence. The dolphin general reveals their secret plan to infiltrate the human world, and he gives her the mission. (The dolphin will totally be a love interest, right? He'll check in with her regularly, to make sure she stays on task.)
The mission is dangerous, because every other mermaid agent has refused to return to the sea. And that's about all I could think of. I have no idea what happens next, or what characters the mermaid would meet.
I did think we could have a grand finale with the Titanic. Say, our heroine learns about the Titanic and is ordered to destroy it by any means necessary. That's her final mission, to test her loyalty.