As you see in the opening cinematic, Jake will talk for a bit about how great it is to be a dog. Then, he will introduce Daisy who gets caught by Wayne and Dwayne, and Jake goes to save her, but they catch him too. Luckily for Jake, they didn't close the door of the van properly and he falls out the back.
With control of Jake, your goal is to try and save Daisy. You begin the journey in the small town of Clarksville, and you'll need to collect bones to bribe your way past the Doberman belonging to The Dog Catcher to get to the different areas.