"Muscles" is a relative of Jerry's and gets the call when Tom is being unusually nasty to the mouse we all know. Muscles, a tough guy with a bowler hat, lives elsewhere and is so tough he beats up all the alley cats and is feared by them all.
Once he arrives at Tom and Jerry's place he wastes no time with our famous cat. He tells him plain as can be to stop messing with Jerry....or else. Tom realizes he's in for a battle and immediately begins to train with weights to build up his muscles.
The battle is on. Need I remotely suggest who wins?
Regardless of winners and losers, this was a very entertaining cartoon and actually not as violent as I've seen in many other T&J cartoons. The storyline is just that, but I thought the comedy routines, just as Tom literally turning into ten bowling pins and then back into a cat, were emphasized more than anything.