The Empire comes into the mix when the Ghost catches a message slipped into the Imperial propaganda machine: a prominent Clone Wars Jedi has been arrested and is awaiting trial... or more likely, a secret execution. Kanan proposes a rescue mission, partly because he hates the thought of losing another Jedi Master, but mostly because she might provide the structure Ezra needs. The bad news is she's being kept in one of the most secure Imperial prisons in the galaxy, which is a less-than-ideal battleground even when the protagonists fully trust each other.
Even worse (for the Ghost crew, not the audience,) is that the Empire is fully prepared for an attack. That "message" the heroes received was little more than a ruse intended to root out hidden Jedi, orchestrated by Rebels first major villain, The Inquisitor. While the show doesn't offer a lot of background information, he's effectively introduced as a force that might beat the Rebels at their own game. It certainly doesn't hurt that he's the first glimpse of an Empire that's clever and genuinely deadly, instead of bureaucratic and stupid. And while he certainly has an air of Sith gimmickry about him (like a dual-blade lightsaber that spins itself for no reason) the presentation is so good that it really doesn't get in the way. I absolutely look forward to seeing more of him.
The most encouraging detail to me however, is that there's so much to love here without feeling like the episode is packed full to bursting. The episode flows wonderfully from training sequences, to internalized self-doubt, to mission preparation, to flawed execution without heavily relying on exposition dumps. Even though secondary characters are moved to the background, they still have ample time to shine without derailing the episode's tone. Rebels even squeezes a little "adult" humor in, somehow stating outright that alien creatures are attempting to mate with the ship without feeling jarring or out-of-place. That's quite the achievement, and makes me excited to see what's coming next.
Bottom Line: "Rise of the Old Masters" is the benchmark Rebels episodes should aspire to, validating the show's entire existence up to this point. It balances a great premise, clever character development, a surprise twist, and the introduction of a deadly villain. While "Masters" has a few minor flaws, it's so well-paced and presented that you'll barely even notice them. Star Wars fans new and old should find much to enjoy here, and will be excited for what comes next.
Recommendation: This episode is worth starting a Star Wars TV series for. Check it out.