Morrisey and Dunbar play British conmen in Chicago, who lift a suitcase with money, and realise it's the down-payment for a hit on a mob boss. In the hopes of more, they pretend to be the killers: needless to say, neither their employers, nor the real assassins, are exactly delighted. Toss in a peeved and pregnant wife, the daughter of the target hooking up with one of her father's potential killers, and you're obviously deep into, "Yeah, right..." territory. Plausibility is not this film's strong suit, unlike, for example, Lock, Stock, which had a logical flow to it; there are too many big leaps here, which limit the credibility.
On the plus side, Morrisey and Dunbar are great; you could build an entire series of movies around their adventures and affectionate bickering - they've known each other since drama school, and it shows. You can tell the script was written by a Brit (Tony Johnston), given the amount of sarcasm in there - we particularly liked the suggestion by Dunbar, that they should use a microwave as an assassination tool, in the hope of awakening a dormant tumour in their victim. In contrast, most of the American characters don't have the same entertainment value: Wahlberg is particularly bland as one of the American killers. Still, the decent supporting cast (including Pete Postlethwaite and Amanda Plummer) help this avoid sinking into tedium for very long.