The "river" of the title is the Hudson, and the "rhythm" is provided by Bob Somers (Crosby), a talented but unmotivated tunesmith, and Cherry Lane (Mary Martin), a clever lyricist--both of whom ghost write for Oliver Courtney, a famous songwriter whose inspiration left him when his wife
ran off with another man, though he prefers to think of her as dead. After meeting at a quiet resort owned by Bob's uncle, the songwriters decide to make a go of it on their own, but Oliver warns them its a tough road for unknowns, which they discover soon enough. Cherry turns to singing in a
nightclub and Bob returns to the farm. Pressured to come up with material for a new big-bud...