Onderhuids tells the story of Keesje, an introvert 15-year old. She and her friends, Lize and Dunja, swim together in a synchronized swimming team. From the outside it all looks like the perfect friendship, but behind the surface hide continuous tensions between the girls. Keesje is constantly afraid of being rejected and longs to maintain her position in the group. While the team is training for the National Synchronized Swimming Competition, Keesje tries to hold her own within her group of friends. She allows herself to be dragged into the vicious behavior of Lize and Dunja, but is gradually becoming alienated from herself. While Lize and Dunja are good with words, Keesje’s strength is in her physical. While the pressure for the match increases, Keesje loses sight of who she is. She constantly fears she will lose out. Although she tries to suppress her frustrations, an outburst is the inevitable consequence.