An American lawyer from Boston comes to town and Angela Black, a detective inspector with Cambridge police, is detailed to observe him. He is under surveillance by the FBI as a possible disciple of Agnus Dei, a fanatical Christian group threatening to assassinate members of America's power elite. Things escalate quickly when, two days after arriving in Cambridge, the lawyer vanishes. D/I Black's pursuit of the truth about him takes her to London, across the Atlantic to Boston, and eventually down to El Salvador, where she pays a terrible price for breaking the rules.