Definitely a minor Pixar effort here. We wisely waited for the DVD to percolate to the top of the Netflix queue.
The plot: you know the massive meteor that (in our reality) hit Earth and caused the extinction of the dinosaurs? Let's postulate it just grazed the atmosphere instead, causing the great lizards to look up briefly, think Huh! What was that?, and go back to their dino-lives.
Millions of years pass, evolution does its thing, and now dinosaurs have invented language, tool-making, and agriculture. But no machinery, they rely on their brute strength to clear their fields, plow the earth, and build rudimentary structures.
Into one such farming family is born Arlo. He's relatively small, clumsy, and cowardly, but loved anyway. It's what dinosaur families do. But Dad's patience is tested when Arlo can't manage to do one simple thing right: kill a pesty "critter" who's been stealing corn from their farm. In the effort to "man up" Arlo, Poppa unthinkingly leads them into a dangerous situation, and before you can say "hey, they're not going to go all Lion King on us, are they?" … yeah, that's exactly what they do.
So Arlo is now small, clumsy, cowardly, and under extreme levels of guilt. This causes him to go off after the "critter", which is (well you can see from the DVD picture over there) is a cave-boy. Yes, evolution has allowed a more-or-less-human species, although they seem to have a lot of canine qualities too: howling, panting, scratching… (The critter eventually gets named "Spot".)
Arlo's critter-bashing quest quickly goes awry, and he finds himself lost, and his only option is an uneasy alliance with Spot. And… well, I'm pretty sure you can figure out the general outlines of the plot from there. It's pretty generic.
Pixar renders the dinosaurs cartoonishly, which is a little surprising. They appear to be made of the same rubbery stuff as their sold-at-WalMart action figures.
But (on the other hand) Pixar has gotten really good at environments and landscapes. Some of the scenery seems photorealistic and breathtaking.
And there are a few indications that there's still brilliant cleverness at Pixar. Arlo and Spot encounter a seemingly-wise-but-actually-confused Styracosaurus named "Pet Collector" who delivers a short but hilarious bit of dialog. (Spoilers in IMDB's "quote" section if you must, but it's almost worth sitting through a lot of the movie's sentimental dreck instead.)