The African Queen
The rough-hewn captain of a broken-down East African riverboat teams with a straitlaced missionary to take on a German gunboat during WWI.
After a religious spinster's (Katharine Hepburn) missionary brother is killed in WWI Africa, dissolute steamer captain (Humphrey Bogart) offers her safe passage. She persuades him to use his riverboat to destroy a German gunboat. The two spend most of their time fighting with each other rather than the Germans. Time alone on the river leads to love.
It is a picture with an unassuming warmth and naturalness...