You know what you're getting with Alvin and the Chipmunks: squeaky-voiced little cgi chipmunks who get in trouble for the amusement of your child. I'm scoring this based on children's amusement, because no adult would watch this on their own (I hope).
Both the Chipmunks and the Chipettes, plus the hapless Dave, go on a cruise ship for a combination holiday and awards ceremony. (Don't ask.) They cause trouble, with some predictable slapstick moments, plus some singing and dancing.
Then they all get lost overboard and wash up on a desert island. ("Chipwrecked", geddit?) Dave is separated from them, along with the evil record producer guy from the earlier movies, who for some reason is dressed as a giant bird. (Don't ask.) So the Chipmunks have to look after themselves. And here comes the moral of the story: Alvin has been told off for being irresponsible, so now he has to take respnsibility and try to sort things out, because sensible Simon has been turned crazy by a spider bite. (Don't ask.)
To cut a short story even shorter, there's some mysterious buried treasure, a volcano, some chipmunk-romance, and more slapstick and dancing. Oh, and a really bad gag based on the old Tom Hanks movie Castaway. (Don't ask.)
Anyway, by the end there's been some character development, everyone is happy, safe, and off the island. And then there's a big musical number. Beause, you know, just because.
Is it any good? Well, kids enjoy it, so that's the main point. As an adult, I didn't hate it, which is okay. I'd not want to have to see it again, though.