Selkies are able to transform to human form by shedding their seal skins and can revert to seal form by putting their selkie skin back on. Stories concerning selkies are generally romantic tragedies. Sometimes the human will not know that their lover is a selkie, and wakes to find them gone. Other times the human will hide the selkie's skin, thus preventing them from returning to seal form.
A selkie can only make contact with one particular human for a short amount of time before they must return to the sea. They are not able to make contact with that human again for seven years, unless the human is to steal their selkie's skin and hide it or burn it. (Wikipedia)