Merchant Iivari encounters significant challenges in his entrepreneurial career when Merthen, a commercial counselor, attempts to prevent his success. Merthen's daughter Verna falls in love with the honest Iivari, who is not acceptable as a son-in-law to the Merthen family.

Engineering student Petteri and retired captain Jere Renkonen arrive in Terijoki as guests of architect Matti Marras at the villa he has rented. In the midst of everything, the owner of the villa, Russian émigré prince Asatiánts, arrives with his niece Darja. Petteri eavesdrops and learns that the prince is expecting a "man from Marseille" at the villa. This man is supposed to take a secret document hidden in a large cabinet in the hall across the border to Moscow, containing information about the hiding place of the prince's family jewels.

A French cultural research group arrives in Finland. From Helsinki, the group travels to Saimaa to participate in Midsummer rituals, where the summer night celebrations overwhelm the researchers.