Blue My Mind is a 2017 movie from Switzerland.
A small girl stands by the sea, her parents nowhere to be seen.
Years later, Mia starts a new school after her father got a new job and had to move. Mia tries to get into good standing with the cool group. She plays a tough girl and gets noticed by the group leader, Gianna. Mia occasionally has cravings for salt water and fish. She even impulsively eats the fish from her mother's aquarium.
During her first period she notices changes, like her toes begin to fuse together. At first Mia visits a doctor, but she feels that she can't get any help there. In an attempt to be full accepted by the "cool girls," she shoplifts and loses her virginity. Not wanting to be different, Mia cuts the extra skin from her toes herself. Later she discovers that her legs appear bruised on their own, and even her belly button has disappeared. Mia feels convinced that she was adopted, due to the changes and the lack of any photos of her mother being pregnant. Her mother not wanting to talk to her about it doesn't help.
Gianna almost drowns at one of the night parties. Mia jumps in after her and saves her. Mia then discovers that she has grown gills on the sides of her torso. Mia's legs turn darker and darker, and she tries to hide it from everyone, but Gianna catches sight of them. Not knowing what is happening to her, Mia start spiraling and abuses drugs and alcohol. At the next party when Mia is totally wasted, she goes into a room with group of men for pleasure, and when Gianna comes for her, the whole party ends up discovering that something is wrong with her legs. Mia runs back home and hurts herself with a knife.
The next morning, Mia wakes up on her kitchen floor with a tail. She drags herself to the bathroom to hydrate. After flooding the whole apartment, Mia decides to call Gianna. Gianna moves Mia to a truck and drives her to the sea. Mia thanks her for everything and goes into the sea.