Diana Nyad (Annette Bening) is not a simple woman nor one who gives up easily, especially after having lived her entire life dedicated to swimming. Now, the long-distance swimmer faces a new challenge, one of the most important of her existence. At the age of sixty-four she wants to travel the 177 kilometers that separate the coast of Cuba and Florida without any type of support and help. Many consider it a practically suicidal mission, but she will be supported by her best friend and trainer Bonnie Stoll (Jodie Foster).