Puggy is a dog.
I need to make sure you understand that is no judgment call, it is a statement of fact. He is a dog, of the breed called the 'Pug' (big surprise there, I'm sure). He was named by his master, Bill, a young boy who is very fond of Puggy but is not exactly original when it comes to thinking of dog names.
Bill and his parents live in a typical suburban neighborhood so obviously Puggy does to. He has the benefit of a doggy door to the backyard, which is fenced for protection. Luckily Puggy has long ago made his own hole beneath the fence which has either gone unnoticed or is relatively new. Whichever it is, it gives Puggy an opportunity to go on walk-about whenever he wants and apparently he wants it a lot.
This explains how Puggy became involved with the aliens - those creatures which, Puggy would come to learn, came to this planet for some not very nice reasons and took human forms which we are old is "difficult to get away with".
Now normally this would likely be the end of it. Puggy may have found out by snooping and being in the right place at that right time to overhear one of the aliens talk to one of the several neighborhood pets that he had so far kidnapped and was experimenting with in his garage lab - but what could Puggy do about it? He was a dog, and a little one at that - though Pugs are notoriously persistent.
But in investigating by sneaking into the garage, he pushed a bunch of buttons on the computers in the lab and an odd cylinder marked "modification" fell near him. Puggy snatched it and beat a hasty retreat. I'll leave to you to learn how the object got switched on and somehow "molded all over his body and then absorbed into his skin".
After that, well strange things started happening, beginning with little pole rising from his head which shot laser beams and somehow Puggy is able to fly and, well lots of other stuff and suddenly the aliens here for really bad things had someone out stop them because Puggy, already a pretty smart dog for - well, a dog - was all of a sudden a whole lot smarter.