In the tranquil warmth of Lake Como, Stefano Ugo Tognazzi and Giovanna Senta Berger spend the summer in a small house by the water. The life of the 45-year-old architect and his younger wife feels steady, predictable, and a little dull, with days measured by routine rather than romance.
One morning a young couple pitches a tent on their property and swims naked in the lake. Stefano's initial indignation fades to curiosity, and he invites the visitors to stay the night, hinting at a partner swap that would shake up their quiet routine.
Yet the couple in love keeps to themselves, and nothing comes of the arrangement. Meanwhile, Stefano betrays his own moral stance by sleeping with his secretary. He grows more insistent, pressing the idea at every turn, hoping Giovanna Senta Berger might eventually agree.
Giovanna, cool and skeptical, resists the idea and even reads the letters in the newspaper's box, sensing that Stefano might be behind them. After she discovers something that seems orchestrated, Stefano admits it. The moment turns their evenings into a dryly amusing pastime as they read the letters aloud together.
Driven by his fixation, Giovanna eventually agrees to try swapping. The first couple they visit turns out to be old and unsightly, and Giovanna laughs at the mismatch while the pair leaves without a qualifying exchange.
On the next visit, Stefano is drawn to the woman, but Giovanna finds the man pushy. Stefano then recalls seeing the woman in a photo as a prostitute, recognizing an attempted fraud, and the man bolts with his supposed companion.
After another unsuccessful trip to a similar club in Ticino, the couple abandons the project for a time.
An unexpected opportunity arises when their friends Diego Silvano Tranquilli and Gabriella Gianna Serra bring them to a grand palazzo. Giovanna is genuinely taken with the cultivated Diego, who plays Beethoven for her on the piano while Gabriella, in a different room, lies with Stefano.
But when Diego's piano goes quiet, Stefano races through the palazzo, unable to find Giovanna in his search for the truth. In the morning Giovanna reappears with the glow of a night she clearly enjoyed, while Stefano is left to grapple with what happened and what it means for their marriage. Diego Silvano Tranquilli lingers as a memory of the evening, and Gabriella Gianna Serra remains a part of the tale as well.
He asks Giovanna to confirm under oath that nothing happened. She agrees, though with a complicating air, providing him a sliver of support as he battles to keep the reality at bay.