Alain van Versch, an unemployed father in his mid 20s known as Ali, arrives in Antibes, southern France, to look for work to support his young son, Sam. Having no money, he crashes with his sister Anna, who already has her own share of problems with money and temporary employment.
Ali gets a job as a bouncer in a nightclub but still keeps his passion burning for fighting. On a usual evening in the night club, Ali meets Stéphanie and escorts her safely to her home after she is injured in a brawl at the club. She works at a local marine tourist park where she suffers a tragic accident during a show and wakes up in the hospital to realize that her legs have been amputated. Ali meets a guy at work who informs him about a kick boxing fixture he can make money from. Stéphanie, now in a wheelchair, is terminally depressed and gives Ali a call. Ali visits her and takes her to a beach where they swim.
Over a period of time, Ali and Stéphanie spend a lot of time together and Stéphanie starts to feel better about herself in Ali's company. She gets artificial limbs and starts to walk again. Stéphanie accompanies Ali to his mixed martial arts fights and is surprised to learn he has a son. After a frank discussion, Ali offers to have sex with Stéphanie to help her adjust to her new body and their friendship then evolves to include casual sex. Ali, Stéphanie, and some friends visit the same night club where Ali used to work. Ali goes to the dance floor and flirts with a girl as Stéphanie watches curiously. Ali goes away with the girl leaving a dejected Stéphanie with other friends. The next day Stéphanie questions Ali about their relationship status. Their intimacy increases and a symbiosis forms between them, with Stéphanie managing Ali's bets for his fighting.
Anna is fired from her job when the managers realize she has been taking home expired food products. Anna blames Ali for this, as Ali was involved in an odd job where he installed spy cameras in work areas at the direction of the management to spy on the activities of their employees. This results in a standoff between Ali and Anna's partner and Ali leaves. Sam stays with Anna while Ali goes to a combat sports training facility near Strasbourg (as evidenced by earlier references in the film and the fact that Anna's partner drops Sam off on the way to a delivery in Colmar), losing touch with Stéphanie. Sam visits Ali at his facility for a day in deep winter and both are shown playing in the snow and on a frozen lake. A weak spot on the frozen lake cracks and Sam falls through the ice, and swiftly loses consciousness, submerged in the icy waters. Ali momentarily distracted by a call of nature, takes a while to realize that Sam has had a dangerous accident. Once he spots the hole in the ice and finds Sam in the frozen lake, Ali releases a desperate volley of punches to break the surface and gets an unconscious Sam out. In the process, Ali fractures his hands.
Sam survives and in the hospital Ali breaks down while talking to Stéphanie on the phone and confesses his love for her. As Ali narrates, explaining how broken bones normally heal stronger than before, but not in the hands, in which the pain is sure to return. Ali celebrates a fight victory in Warsaw as Stéphanie watches. Then he takes Sam by the hand and leads him out through the revolving door of a hotel.

Question: What do Ali and Stephanie do for one another in their symbiotic relationship?
Answer: Ali serves as her sex partner and Stephanie manages Ali's bets for his own fights.