The setting is a prison camp in French Guiana, where we first see Clark Gable released from the darkness of solitary confinement. He's an unrepentant criminal in a world apparently without hope of repentance or even reform. His fellow convicts are a rogues' gallery of heavies. Eduardo (Kali guru from Gunga Din) Ciannelli is a bitter Bible-reader, the wrong kind of Christian, as we'll learn. Albert (Dr. Cyclops) Dekker is a British con with a plan to escape and a reluctance to let Gable in on it. On the outside looking in is Peter (needs no introduction) Lorre as "M'sieu Pig," a professional informant, and Joan Crawford, a drifter of dubious occupation who risks expulsion from the island merely by talking to Gable, who strays from his work detail to hit on her, despite ratting him out when he goes AWOL to visit her room. The guards didn't notice him missing at first because another man, Cambreau (Ian Hunter) actually infiltrates the prison at the right moment to keep the count right. None of the other cons know him but they don't question his presence or his willingness to aid the escape.
Gable misses the breakout because Dekker had knocked him out with his boot in the middle of the night, but takes advantage of the confusion to make his own escape from the infirmary. Cambreau has helpfully left him a map showing the rendezvous point, where a boat will take the fugitives off the island. Along the way he re-encounters Crawford, while Ciannelli is beaten and left for dead by two fellow escapees who resent his hoarding food. They all make it to the beach, however, where Gable beats down Dekker and declares himself in charge. Ciannelli doesn't make it to the boat, however; he dies in an epiphany after Cambreau sets him straight about the word of God. Cambreau has an odd effect on people. His gentle manner at first disturbs, then inspires people. The weird thing it that he inspires some people to die. In a paranoid fit, one con throws the gang's only keg of fresh water overboard because he thinks they're out to steal it from him. After a few gentle words from Cambreau, he jumps into the sea to retrieve the keg and gets eaten by sharks. Feeling bad over the death of his protege (the Sale novel apparently hints at more to the relationship), Dekker, under Cambreau's influence, volunteers to taste-test the recovered keg for leaked sea-salt. One swig proves a death sentence. Of the escapees, only a smug Bluebeard type (Paul Lukas) seems immune to Cambreau's spirituality, though Gable and Crawford struggle hard against it, as they do with their attraction to each other.
The film builds to a climax that modern viewers might agree would have seemed blasphemous in 1940. The boat has reached safety, and after Lukas has gone his way and Gable has left Crawford behind, our protagonist and Cambreau sail out again into a powerful storm. Throughout the picture, Gable has been a mocker of Christianity, the kind that knows the Bible and can pick out its weak points, who scoffs at the idea that man was made in God's image, who uses the Song of Solomon's description of the human body as a "temple of God" as a come-on line to Crawford. On this new boat, he's finally had enough of Cambreau, and during a struggle Cambreau falls overboard. A triumphant Gable has a Colin Clive moment. As Cambreau bobs helplessly in the waves, his arms symbolically draped behind him, cruciform, across a piece of driftwood, Gable mocks him with possibly the most over-the-top acting of his career. "I'm your God, now!" he roars at Cambreau. In a pantheistic fit, he points to the boat captain -- "He's God!" -- and to himself -- "I'm God!" But when he takes the obvious next step, telling Cambreau, "You're --" he suddenly stops as if he's suddenly realized something. He gets it! -- whatever it is -- and dives into the raging sea to save Cambreau. From there, a final reconciliation with Crawford (and a final rebuke to Lorre, who's been lusting after Crawford throughout) is inevitable, if its consummation will be deferred until Gable serves out his prison term, as the Code required.
One of the old-time conventions of cinematic Christianity is the idea that there's something about the mere presence of a true Christian (and let's face it, Cambreau is something more than a mere Christian believer) that affects people whether they like it or not. The Christian's implacable serenity has a compelling power -- or repelling for exceptions like the Lukas character, who jokingly identifies himself as a son of Satan -- like that of Dracula when he gets into his pitch about the strange twilight world. The Christian exists in a state of peace that is implicitly available to anyone and desirable for nearly everyone. Unless you understand this concept a lot of the action of Strange Cargo will make no sense. To be fair, it's not so much Christian propaganda as it is a story that takes a lot of Christian mythos or sensibility for granted in a way we don't expect from Hollywood movies now. Reviews of the novel Not Too Narrow, Not Too Deep, suggest that Sale sustains suspense for a while over whether Cambreau himself is angel or devil, but in the movie his affiliation is unmistakable from the start, despite the sometimes-fatal consequences of his ministrations. Borzage, an arch-romanticist of the screen, no doubt intended Strange Cargo as an uplifting experience, but something that seems old-timey now struck certain powerful people as unacceptably unorthodox in 1940. The film retains some interest today despite its questionable theology as a showcase for Gable, who plays as unlikable version of his usual rogue character as he could get away with for much of the picture, and a jamboree of character actors, even if most of them go soft along the way. One thing's for certain: they don't make 'em like that anymore.