Devlin is an Irishman and in his youth he was a terrorist but now he is a famous writer and officially he is a supporter of an American society proposing to help the victims of terrorism.
Actually he is buying from Pauley weapons for the terrorists. Devlin kills him because he think he has been betrayd. Lt. Columbo is responsible for the inquiry and becames friendly with Devlin.
But it is very dangerous to be familiar with Lt. Columbo when you are a murderer.
