For fans of quality drama, Sunday at 9 p.m. has become a special kind of logjam. Yet amid an embarrassment of riches consisting of CBS’ “The Good Wife,” AMC’s breakaway hit “The Walking Dead” and HBO’s still-mesmerizing “Boardwalk Empire,” only Showtime’s “Homeland” can boast a best-series Emmy on its resume.

All of which might help explain the hand-wringing over the perceived decline in quality for “Homeland,” which has been intermittent, but steady, since about the midway point of the show’s second season. The drumbeat became particularly loud after last week’s episode (and WARNING: Spoilers lie ahead), when the series revealed the entire year’s plot to this point has been an elaborate red herring, designed to convince Iranian operatives that disgraced CIA agent Carrie Mathison (Claire Danes) could be turned into a counter-spy.

Still, after watching this week’s episode — which saw Carrie plunge further into that scheme, while continuing to follow what might be the worst “B” plot in the history of quality drama involving pouting teenager Dana (Morgan Saylor) — it seems unlikely “Homeland” can fully snap out of its doldrums. In other words, those complaining that the show isn’t approaching the operatic heights of its first season had better get over it, because at this point, it looks like it is what it is.

That’s not to say Sunday’s episode was without its pleasures, from the aforementioned Iranian enjoying a hamburger to the pointed speech delivered by Saul (Mandy Patinkin) after a Dick Cheney-like hunting excursion. Patinkin’s role has been wisely beefed up to compensate for the character of Nicholas Brody (Damian Lewis), the U.S. soldier-turned-terrorist-turned-Congressman-turned-fugitive, left missing in action for the fourth time in five episodes.

Yet “Homeland,” ultimately, has demonstrated itself to be a show without much of a second act. The game of cat and mouse between Carrie and Brody was fascinating. The moment where she broke him down under interrogation was as good as anything on TV last year.

After that, strictly in creative terms, the series had clearly peaked. Because now it’s pretty familiar spy-versus-spy stuff, featuring a stellar cast and taut situations, yes, but seemingly unable to re-ascend to anywhere near to the lofty creative perch it occupied.

As compare-and-contrast exercises go, the premise of “Homeland” simply isn’t as resilient as something like “Walking Dead” — which has chewed through major characters and kept reloading with new ones — or “Boardwalk,” which slowly, inexorably builds season-long arcs around new heavies, allowing some of its major players (such as Kelly Macdonald) to recede into the periphery without viewers feeling the loss as acutely as Brody’s absence.

Sunday’s slow-going “Walking Dead,” in fact, offered a solid demonstration of how effectively the show has replaced its casualties, featuring the characters of Daryl (Norman Reedus) and relative newcomers Michonne (Danai Gurira) and Tyreese (Chad L. Coleman). Moreover, after the macro threat posed last season by the Governor, the plot has self-consciously turned to a micro one — literally, inasmuch as the major villain has been a virus, wreaking havoc on the prison where survivors of the zombie apocalypse have taken refuge.

To be fair, the year is still young for both “Homeland” and “Walking Dead.” Still, if there’s been a real standout among these cable heavyweights thus far give the nod to “Boardwalk,” which has expanded the role of the fabulous Michael Kenneth Williams and introduced Jeffrey Wright as the mysterious Valentin Narcisse, setting up a power struggle that paid off in an extraordinarily tense and visceral sequence (my face still hurts just thinking about it) in the episode that premiered Oct. 27.

On the eve of “Homeland’s” second-season finale, I suggested those responsible should recognize the corner into which they boxed themselves by making the third season its last: “It’s hard to imagine a scenario where the program could run much longer and remain plausible without completely hitting the ‘reset’ button,” I said, adding, “Not every series is built to sustain itself for five years (once the ‘We’ve made it!’ benchmark for syndication), much less seven or eight — especially today’s wildly intricate serialized dramas.”

Of course, that won’t happen. “Homeland” remains a success, and in TV that’s never something to be tossed away cavalierly, which explains why it’s already been renewed. Similarly, with the astonishing numbers “Walking Dead” has been posting this month, don’t be surprised if Robert Kirkman’s creation is still knocking ’em dead when Rick’s baby is all grown up and every bit as much of a zombie-killing badass as big brother Carl.

So those grousing about “Homeland” would appear to have one of two options: Accept the show on its current terms, or move along. Because while there’s still a potentially enjoyable franchise here in the mode of “24,” there’s seemingly no way to reconstruct the magic the series initially conjured with a premise that was too intricately woven to hold together indefinitely.

Put another way, as a native of Nucky Thompson’s home state of New Jersey might say, in today’s TV landscape of serialized, novelistic drama, not everything was born to run.