Splinter
We flash back to right before the Smallville meteor shower to a plucky young rat practicing kung fu he learned from his master at the Smallville Community Center. The meteors come down and destroys the building but somehow the rat survived.
Fast forward to current times, Clark is doing his good guy stuff but keeps finding that someone else has been beating him to the scenes of crimes. He and Pete later follow a trail of rat fur down to the sewers of Smallville (?). They find a strange lair with ninja weapons mounted on the walls and pizza boxes littered everywhere. The episode ends as we see a man-sized rat approach Clark from the shadows.
Pete: “Damn, who’s that?”