Ship wrecked

[Katrina]

Megan was on a ship lost at sea and managed to drift to a desert island.  She is alone on the island or so she thinks.  She finds fruit trees and a cave she thinks is empty to sleep in.  What resides in the cave can be a  Sasquatch or tentacle monster and it decides to make Megan its pet.

Megan slowly realizes after a couple escape attempts the monster knows the island and has killed to keep it, well deserted, and she was lucky enough that it kept her alive for its pleasure rather than kill her and she has to depend on it to keep her alive.