Lino Ventura plays Milan, a hit-man, who is usually good on what he does. This time he is hired to kill an important witness in Montpellier. He rents a room in a hotel opposite the court building. Unfortunately, the guest next to him, François Pignon (Jacques Brel in a bad haircut) is trying to commit suicide after his wife refused to meet him. Milan prevents the bellhop to report this to the police and promises to take care of Pignon. We will see later that if the bellhop reports the suicide attempt to the police, the police will come and check their IDs - and Milan doesn't have one. This is stupid as fake ID can always be bought - like in other movies. I think Milan should have killed both the bellhop and Pignon if he wanted to be left in peace to work - but Milan doesn't do that. I guess this hit-man only kills his target. To change room is also impossible, as perhaps the one he has chosen has a perfect view - or the hotel is fully booked.The English title 'A Pain in the Ass' is very true. Instead of preparing himself for the mission, Milan finds himself driving Pignon to see his wife in a ranch, and driving a pregnant woman to a maternity hospital. Later, a doctor who lives with Pignon's wife injects him with a tranquilizer. In short, it all goes wrong.
Both main actors are good. Lino Ventura portrays a tough hit-man, who although is very annoyed with his new "friend", maintains to have a calm face, but inside burst with rage. Jacques Brel is as annoying as possible, but doesn't know it, and thinks that Milan really cares for him. There is a good scene where both main characters prepare themselves to kill in the hotel rooms: Milan: the witness- who will soon arrive, while Pignon: himself.