The plot is set in motion when Barry Kane (Robert Cummings) and Ken Mason (Virgil Summers), labourers at an aircraft factory, encounter uptight co-worker Frank Fry (Norman Lloyd), who automatically provokes suspicion for carrying large amounts of money on his person ("So that's what they look like," Kane murmurs appreciatively as he picks up a dropped hundred dollar bill). Moments later an explosion tears through the factory. Kane and Mason try to tackle the blaze. Kane hands his friend a fire extinguisher. Neither of them realise it's been tampered with and is now full of gasoline. Mason dies in agony and Kane finds himself the chief suspect. Fry, meanwhile, has disappeared.
Remembering an address on an envelope Fry dropped along with the money, Kane sets off cross-country to track the down the Nazi agent and clear his own name. En route, he is assisted by a cop-hating truck driver, a blind musician and his circumspect daughter Patricia (Priscilla Lane), and a travelling circus troupe. The trail leads from a picturesque ranch to a ghost town to the home of a wealthy socialite to a dockyard and finally to a confrontation atop the Statue of Liberty.
'Saboteur' is vintage Hitchcock: Kane and Patricia are a mismatched couple on the run, a la Robert Donat and Madeline Carroll in 'The 39 Steps' (there's also a moment heavily redolent of that film where Kane suddenly realises that an ostensibly helpful and sympathetic character is actually one of the bad guys); there's the stiff-upper-lipped-Brits-vs-foreign-agents solidarity of 'The Lady Vanishes' re-imagined as American-patriots-vs-Nazi-sympathisers as various characters rally to support Kane in his quest for the truth.
The pace and structure - a series of well-staged set-pieces speedily connected, often with little regard for exposition or narrative logic - both evokes Hitch's British thrillers of the 1930s and looks ahead to the likes of 'North by Northwest'. Like 'Saboteur', it's a chase movie, it's chief pleasure is its half-dozen or so set-pieces (the mechanics of how the protagonist gets from one set-piece to the next being, by and large, inconsequential); also the Mount Rushmore finale is explicitly prefigured in 'Saboteur'.
Where 'Saboteur' doesn't quite scale the heights of first-rate Hitchcock is in the casting: Robert Cummings is a one-note, clench-jawed hero, lacking the ironic panache of, say, James Stewart or Cary Grant; there is little in the way of romantic tension between him and Priscilla Lane, who, while a game heroine, is neither as engaging as Margaret Lockwood in 'The Lady Vanishes' or Teresa Wright in 'Shadow of a Doubt', or as glacially cool as Grace Kelly or Eva Marie Saint in the later films.
Still, there is much to enjoy: the honest joe trucker spouting "say buddy, didya hear about"-type dialogue like a dime-store Homer; the circus troupe taking votes on whether to turn Kane in or not (a nice little irony: transients and outcasts as a microcosm of democratic society); Kane inadvertently infiltrating the Fifth Columnists' organisation; an excellent scene of subtle tension and social hypocrisy as he tries to escape their clutches at a high-society ball; a shoot-out at a cinema, the comedy of the film onscreen an obvious but effective counterpoint to the villainous Fry's lack of compunction at injuring or killing innocent bystanders.
Personally, though, what I find most intriguing about 'Saboteur' is the thematic dichotomy that runs through every scene: as a propaganda piece, it warns that enemy agents can be lurking anywhere, from a press junket to the home of a bastion of society who publically organises fundraisers to that most American of enterprises, a ranch - and yet Kane, identified by the media as a suspect in an act of sabotage against the war effort, his description circulated, is assisted at every twist and turn by citizens who demonstrate their patriotism by aiding and abetting a fugitive.