When his fiancée is murdered by terrorists, a Central Intelligence Agency (CIA) cryptographer named Charles Heller blackmails his superiors into sending him on a field assignment into Czechoslovakia to assassinate those responsible. Just as he enters the field, the CIA locates the stolen material he had been using to blackmail the agency, eliminating their need to cooperate with him. So he is behind the Iron Curtain, inadequately trained, and without support.
He nonetheless reaches his safe house, meets his local contact, Elisabeth, and kills two members of the terrorist band. He discovers the location of their ringleader, Schräger, the man who had killed his fiancée, but attracts the attention of the local security forces, who begin following him.
At the site where he expects his final showdown with Schräger he is ambushed by his CIA trainer, Anderson, who explains that Schräger is a double agent working for the CIA who had killed his fiancée under orders to establish his bona fides by murdering an American. He insists that Heller explain his own rogue status to Schräger so that the terrorist, who believes Heller was sent by the CIA to kill him, will continue to work with the Americans.
A senior agent of the local security service, Lakos, overhears all this.
Schräger arrives, kills Anderson in a shootout, and is killed by Heller. Lakos, who has come to respect Heller, helps him and Elisabeth escape back to the west in exchange for his promise to write a book embarrassing the CIA.
