Mickey Rooney has been disgracing papa Henry O'Neill by hanging out in night clubs instead of the Yale Quad, so he ships him out west where he encounters Judy Garland. There are some subplots, but it's mostly about Miss Garland singing Gershwin songs and Rooney mugging.
It's a transitional musical from the Freed unit. MGM had been doing musical extravaganzas in the 1930s, and operettas starring Jeanette MacDonald and Nelson Eddy. The Arthur Freed unit was evolving the musical into the new Broadway style, in which the songs advanced or expounded on the plot, instead of stopping the horse race while Jolson did bird imitations. This one has some big musical numbers involving the Tommy Dorsey Orchestra, and it has some numbers that comment on the characters' states of mind like "Could You Use Me?", and the lovely "But Not For Me". It also has numbers more suited for a revue like "Biding My Time." So musically it's a mixed bag, except for the fact that it's all Gershwin tunes.
You can't fault any music by George Gershwin, but you can raise your eyebrows at some of the self-indulgent lyrics by Ira Gershwin, full of all-too-clever feminine rhymes instead of honest sentiment -- and some unfortunate arrangements, like an orchestral swing version of "Fascinating Rhythm" that reduces the rhythm to nothing under the Dorsey syrup.
Even so, the enormous energy and talent of Rooney and Garland carry this movie easily over the finish line, and if the production number of "I Got Rhythm" is directed by Busby Berkeley is over the top, the starring pair triumph over its rigorous and meaningless spectacle.