Bogart's dishonorably discharged (so we're supposed to think) from the army, and he soon finds himself on a freighter bound for the Panama Canal. Actually, he's undercover, trying to stop the Japanese from blowing up the locks at the canal and blocking it as a means of access to the Pacific. On the freighter are Greenstreet, a Japanese sympathizer, and Astor, who's not quite what she seems. All three give fine performances, and there are some funny lines (as when Bogart and Greenstreet pull their pistols and Bogart says, "Mine's bigger than yours").
While it's fun, it's not in the league with the Falcon. Why not? The plot is more predictable, for one thing. For another, there's a bit of racism that, while perfectly acceptable to audiences in 1942, considering what was going on then, is likely to bother viewers now. If you can get past those things, you'll have a good time.