In the previous Antoine Doinel film, Bed and Board, the marriage between Antoine (Jean-Pierre Léaud) and Christine (Claude Jade) had survived Antoine's infidelity. Love on the Run is set eight years later when Antoine is over thirty. Having an affair with Christine's friend Liliane (Dani) and divorced Christine, he gets a job as a proofreader, and falls in love with Sabine, a record seller. He also writes an autobiographical novel. He meets Colette (Marie-France Pisier), his teenage love who had appeared in Antoine and Colette, and who is now a lawyer. They impulsively go on a train journey and read Antoine's novel. However, later follows a meeting between Colette and Christine: His romance with Sabine is as complicated and unstable, and it turns out that two of his former flames, Christine and Colette, play a pivotal role in helping him find happiness.
Source : Wikipedia