Arch of Triumph centers around two key characters, Ravic and Joan. Ravic, a talented surgeon whose true name is Ludwig Fresenberg, served in World War I where he encountered Sergeant Katczinsky. He hid two friends from the Gestapo, was captured and harshly interrogated by Haake, and subsequently sent to a concentration camp. Ravic managed to escape and became a refugee in Paris, living without a passport or a country. He saves Joan Madou from suicide following her lover's death, and they fall in love, each finding in the other a stabilizing force in a rapidly changing world. However, like the fleeting Paris lights and the fragile peace, their romance is doomed by the looming threat of war. Their relationship begins to unravel when Ravic, after assisting a severely injured construction worker, is questioned by the police and deported due to his lack of valid documentation.

Three months later, Ravic sneaks back into Paris only to discover that Joan, who often claimed she would be lost without him, is now living with an actor. Deeply hurt, Ravic starts to distance himself from Joan, despite her persistent efforts to rekindle their affair. When Joan is mortally wounded by her new lover, resulting in a bullet lodged near her spine causing gradual paralysis, Ravic spares her from a prolonged and agonizing death by administering a lethal injection when her pain becomes unbearable.

Another significant character is Haake, the Nazi Gestapo interrogator who tortured Ravic and drove Sybil, Ravic's first love, to hang herself in a concentration camp. In an ironic twist, Haake frequently appears in Paris on secret missions, and once Ravic identifies him, he begins to plot his revenge. This secondary storyline injects additional tension into the narrative. After killing Haake, Ravic feels a sense of release, especially from the painful memories of Sybil: "Haake's death had freed Sybil's face from its look of death ... At last he could have peace." Yet, in another twist of fate, Ravic's revenge and newfound peace coincide with the eve of World War II, and the following day, he is sent to a French internment camp.