The first 45 minutes are straight up terrible. It's boring, unconvincing, and just plain stupid. Hepburn is a missionary's sister who ends up on a boat with Bogart, who's meant to be a hard-drinking sleazy sort of guy. They are supposed to not like each other very much. Problem is, Bogart is actually a perfect gentleman, and Hepburn is surprisingly laid back for a prissy prude. Then they actually get into an argument. And then... they fall in love. But what's curious about that is that it actually dramatically improves the film. The chemistry between them is actually really lovely. They do a perfect job as a middle aged couple in the throes of first love. It's touching and sweet and it fills you with joy as they call each other old girl and dear and brave the wilderness together. Hepburn's character, given the times, is a raging badass yet still rather demure, a very nice female character for once. Bogart is a total sweetheart. It's all very nice. By the end though, you can't help but roll your eyes a little bit. I mean, it's just so over the top. The whole adventure plot is completely subordinated to the romance in a really extreme way. The obstacles they face become more and more extreme, and their solutions become more and more implausible, but it's ok! They LOVE each other! Still, it's sort of charming. Probably the greatest scene in the movie is when Bogart does an impression of a hippopotamus and Hepburn laughs with delight.
One does feel compelled to point out though, that Achebe's famed critique of Conrad's Heart of Darkness - or more broadly, of Western depictions of Africa - has rarely been quite so fully illustrated. Africa in this film is nothing more than a symbol of wilderness and adversity. The continent is represented by the landscape, which is basically the unknown, full of terror and danger. The few scenes with actual Africans are mostly of chaotic masses speaking a language that's incomprehensible to most viewers, and behaving basically like a mob (shooting happily at the ship, fleeing in terror, singing discordantly in church). Though here, as in Heart of Darkness, it's worth considering how the other Other of the work figures in. Namely, The African Queen also features lots of Germans, who are either represented metonymically by scenes of destruction, or in groups, speaking an incomprehensible (to most viewers) language and killing stuff. I think there's still a difference between the two, but still, it's worth considering.
Overall - I can understand, I guess, why this movie was so loved in its time, in that it IS Hepburn and Bogart, and a very sweet love story. But time has not been good to this movie, and watching it now, you have to admit that it's pretty ridiculous and over the top, not to mention, it takes a long time to sort of get in the swing of things after a pretty drab beginning.