The report of concentration camp inmate No. 44904 (S), also known as Gérald or Jorge Semprun, expands into an autobiographical portrait. In his memory, various life stages, events, and encounters come to life: emigration, studying at the Sorbonne, his commitment as a resistance fighter, arrest and deportation, secret trips to Geneva and Prague, East Berlin, and the Soviet Union in the early sixties, holidays in Crimea with important officials, discussions with critical communists like Adam Schaff, and confrontations with Carrillo, the former General Secretary of the Spanish Communist Party. Later, the show trials in Prague, where fellow inmates from the concentration camp are also sentenced.

Semprun also recalls discussions among intellectuals in Paris: conversations with Sartre, Simone de Beauvoir, or Yves Berger; he recounts the filming with Yves Montand and Costa Gavras for the movies "The Confession" or "The War is Over."

"What a beautiful Sunday!" is a contemporary historical document, a poetic autobiography, and at the same time, a very personal, passionate, often ironic, and bitter engagement of the "renegade" Semprun with communism.