Theodore feels lonely and completely isolated after his dad moves the family into their late grandfather's cabin at the lake. He's stuck renovating the cabin, and babysitting his little brother, Alex, when what he actually wants to do is practice diving to hone his skill.
When his little brother donates his birthday wish to Theodore, he decides to wish for the thing that he loves the most - becoming a merman. Theodore begins to spend more and more time on the lake, where he discovers that a mermaid named Coralia lives in the lake, a mysterious and beautiful creature who assists him in becoming a merman. However, there is something much more sinister living in the lake, something that could jeopardize everything.