In Zurich, a certain Zimmer (Kurt Bigger) is murdered on the 11 streetcar. A short time later, Sebastien Grenier (Lino Ventura), director of an investment company, hears about Zimmer's death on the radio. Shortly afterwards, he receives a book by Alexandre Dumas in the post, in which a particular page is marked. He goes to a meeting and meets Jean-Paul Chance (Michel Piccoli), who claims to be his contact. In fact, Sebastien Grenier is a French spy who, having been dormant for eight years, has now been reawakened. Grenier calls up an emergency code, to which Jean-Paul Chance answers again. The latter then talks about Zimmer and tells Grenier about certain connections that his wife Anna (Krystyna Janda) allegedly has to left-wing groups; the very ones who have claimed responsibility for Zimmer's murder. Since Grenier does not trust Chance, he turns to his friend at the secret service, Henri Marchand (Bernard Fresson), who is also found murdered shortly afterwards. Grenier decides to contact another agent in Munich to find out what game Jean-Paul Chance is playing. But here, too, a murder prevents the case from being solved. Back in Zurich, a certain Richard (Bruno Cremer) calls Grenier and claims that Chance is a Soviet agent. There is another attack, after which Anna is arrested by the Swiss authorities. Chance secures her release, but when Grenier tries to flee with her to Amsterdam, his wife is kidnapped by masked men and found dead a short time later.