Muriel, newly married and newly orphaned, works as a waitress in a San Diego diner. As she pours coffee and empties ashtrays, she eavesdrops on her customers, the ex-jockeys and trainers of the Del Mar racetrack.

When she begins, secretly, to bet on the horses and, shockingly, to win, she feels strangely unready to share her good luck and its origins with her husband Lee.

Instead it is Lee's brother, Julius, a thief and Korean War veteran – and someone she has only met once – whom she longs to tell, and who has struck a spark of promise and possibility inside her quietly ordered life.

Julius has found himself in Las Vegas, where his gift for gambling leads him to a job patrolling the boards above the casino tables, watching through the cigarette smoke for chancers and cheats. There he meets Henry, a blackjack artist and a man who shares Julius's passions, and his secrets. As tourists gather on roof tops to watch atomic clouds bloom in the desert, Henry and Julius's love burns in the shadows – until one night Henry is forced to flee.

Through the parks and plazas of Tijuana and the bars and beaches of San Diego, On Swift Horses mesmerizingly charts the journeys of Muriel and Julius on their separate quests for freedom, new horizons and love.