The last black-and-white film in the Niskavuori series of movies.

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.

Arrested for a murder he didn't commit, American adventurer Bart Lanigan is about to be deported from Finland on a freighter. In the Helsinki harbor, he himself becomes a target for a hit man but manages to escape back to land. Lanigan contacts Arvo Mäki, a womanizing local con man recommended to him as a man he could trust. Lanigan is in Finland to catch an American millionaire businessman, Leonard Weston alias Lawrence Walker, who has fled the USA with the company's monies and begun investing them in Finland. Looking for the mysterious L.W. character while trying to get rid of various hit men, Lanigan and Mäki travel around picturesque southern Finland, joined by a pretty night club singer Marja Salo who won't leave Lanigan alone and insists on marrying him.