It could have been a better film. It does drag at points, and the central story shifts from Boyer completing his mission to Boyer avenging Wanda Hendrix's death, but Graham Greene is an author who is really hard to spoil. His stories are all morality tales, due to his own considerations of Catholicism, guilt and innocence (very relative terms in his world view), and the human condition.
Boyer is Luis Denard, a well-known concert pianist, who has sided with the Republicans in the Spanish Civil War. He has been sent to England to try to carry through an arms purchase deal that is desperately needed. Unfortunately for Denard he is literally on his own - everyone of his contacts turns out to be a willing turncoat for the Falagists of Spain. In particular Katina Paxinou (Mrs. Melendez) a grim boarding house keeper, and Peter Lorre (Mr. Contreras) a teacher of an "esperanto" type international language. Wanda Hendrix is the drudge of a girl (Else) who works for Mrs. Melendez. The local diplomat, Licata (Victor Francken) is already a willing associate of the Falangists.
The Brits (Holmes Herbert, Miles Mander, and best - if not worst - of the lot, George Coulouris) don't give much hope to Boyer's cause (which he soon grasps may be Britain's before long). Herbert and Mander just retreat behind the official policy of neutrality ordered by the Ramsay MacDonald's and Stanley Baldwin's governments during the Civil War. Coulouris here is a typical Col. Blimp type - always impeccable in his native English diction, he is sharp in showing his dislike for foreigners in general.
The one ray of hope is Lauren Bacall (Rose Cullen), here trying to play her role as well as she can - but she can't really. She's an aristocrat - the daughter of a Press lord. It was Bacall's second film, and (sad to say) almost sank her long career. She does act well, but the spark she showed in her first film was due to the dual effect of starring with Humphrey Bogart and being directed by Howard Hawks. Boyer is a fine actor, but he's not Bogie, and Herman Shumlin is not Hawks. Her next film returned her to Bogie and Hawks again, and her star resumed it's ascendancy.
It's a bleak film (as was the novel). Boyer's mission never succeeds, as he has too many hidden foes all over the place. But the villains are likewise also losers - frequently with their lives.
With Dan Seymour as a suspicious foreign tenant of Katina Paxinou (and the man who destroys her). It is well worth watching to catch the Warner's lot of character actors doing their best given the weakness in direction.

I went in to this movie thinking it was going to be the next Clerks, but left feeling let down. The humor was weak and the characters fairly flat. That isn't to say it was all bad, the idea of the dating service in the grocery store seemed like pretty fertile material, but the director switched focus to the cliche'd "save the Mom-and-Pop store from the evil corporation guy". I felt like if he would have just stuck with the dating service plot, he would have come out with a much more memorable movie. Now, to do the film justice, I am from the Rochester area and loved the way he portrayed Webster. In fact, the best Kevin Smith (of Clerks) homage here was giving props to his hometown. Webster, NY is to Checkout what Red Bank, NJ is to Clerks. The director wisely threw in a date at Nick Tahou's. Trust me, as far as things to do in Rochester, a garbage plate is at the top of the list. I was lucky enough to see this film at the Little in Rochester so everybody knew when the odes to the town came up and appreciated them.