Here we have the basic conflict of an old Hollywood musical. There's the humor, the tap dancing, and just the general sense of fun on the one hand. And there's the sappy garbage and the overly-long romantic dance number in the third act. Luckily, modern technology has brought us the miracle of the fast-forward button.
It's the twenties, and talking pictures are the new big thing. A major Hollywood studio decides to turn one of their in-production silent films into a talkie. The problem is that their female lead has a really annoying squeaky voice. There are a few technical troubles, too. It's up to the male lead, his best friend (read: wacky comic sidekick), and a cute aspiring actress (read: romantic interest) to save the production by turning it into a musical.
There's some great stuff in this movie. The test screening of the Dueling Cavalier, for instance, is hilarious. Especially the whole. "No, no, no," "Yes, yes, yes," thing. If you don't know what I'm talking about, watch the movie. Trust me, it's worth it.
On the other hand, there are long stretches of really boring material in there, too. Like I said, without the fast-forward button, these old musicals wouldn't be worth watching.