After a prank is played three best friends get stuck out at sea, with the guy who played the pranks boat. When they run out of fuel, they decide to peddle to an island in the distance called Mako Island. Getting stuck on the Island with no cell signal, they decide to try and go to higher grounds so they can call from help. When they're passing by a brook with some rocks and waterfalls, one of the girls falls into a cave and the other two decides to go to her rescue. The three girls are trapped inside the cave. They cross another corridor and end up seeing a pool inside the hollow mountain.

They jump into the pool of water deciding to swim out, the pool begins to bubble, and they see the full moon shining through the crater of the mountain. Eventually they swim out of the cave and are found by the local nautical team.

The Next day they discover they have become mermaids when they touch water