A series of random suicides in Rome, Italy are attributed to a heatwave, but a young pathologist named Simona—who is working on a thesis about murders disguised as suicides—suspects otherwise. When a young girl associated with Simona's playboy father ends up dead in another apparent suicide, Simona teams up with the girl's priest brother to prove she was murdered and track down the unknown serial killer.

In 1981, in Grenoble, Mickey, Bertrand, Frédéric, Charles, Louise, and Aimée—brought together by a shared desire to make music—are the members of a band called the “Why Notes.” During a wedding ceremony in a church where Charles is playing the harmonium for the occasion, Bertrand, who has come to join him, notices the groom interrupting the ceremony and running away. A few hours later, just as the band is about to leave for a concert in Paris at the high school they attended, Bertrand runs into Clara, who suggests they run away together and then disappears... Bertrand, completely captivated by Clara, leaves the band and sets off in search of her... in Paris.