The Merkind and Human race have never been on good terms with one another. Always the Humans would end up driving a ship through their kingdom, or dropping waste in the ocean that the Merkind would always have to clean, in order to provide the safest place for Merkind, as well as keeping the peasants unaware of the Humans' exsistance.
One day, two human children fell into the water, babies. A mermaid widow quicky came and swooped them up into her arms, bringing them to the castle. They were five years of age, so the queen erased their memories and the king let them have a tail.
He then announced these two as their sons. The kingdom rejoiced while the parents of the children thought they were dead.
But, oh was she wrong.