What's fascinating about Homeland, which is returning this Sunday on Showtime, is the way that its main character's journey so closely parallels the show's. Carrie Mathison (Claire Danes) is brilliant, intuitive, and deeply passionate about her work. She's also prone to horribly bad decisions that back her into corners she can't find her way out of. It's a common idea that TV shows take on the personae of their main characters, but, c'mon, Homeland, this is a little on the nose.

This has never been clearer than it is at the start of the show's fourth season. Much of what has always been good about the show is still good — perhaps even better. But the bad decisions that began to weigh it down in its second year and took their toll in season three cling to it like moss. Homeland keeps trying to rise above itself, and there are thrilling moments in all three episodes sent to critics when it does.

But then Carrie Mathison takes center-stage again and you're reminded all over again of just why this show got into so much trouble that it had to completely reboot its premise just to save itself.

A complete reboot

The most important thing you need to know starting out season four of Homeland is that Nicholas Brody is completely and truly dead. The show doesn't quite haul out the Munchkin coroner to sing a little song about it, but it might as well. Carrie even gets a little monologue in the second episode about how much she misses him, and as far as balancing grieving a hugely important character with moving on to other things goes, the show does a solid job.

The most important thing you need to know is that Nicholas Brody is completely and truly dead

But considering how much of the show's premise was wrapped up in the endlessly vacillating Brody, it necessarily has to do something completely different with the season. That necessitates sending Carrie and the other characters to the Middle East (though they're scattered across several locations as the season begins) and it means digging into a brand new spy story where nothing is as it seems.

A bombing run (signed off on by Carrie) early in the season premiere, meant to take out a terrorist, succeeds in hitting its target — but it also kills almost everyone else at the wedding he's attending. And the more that the characters dig into the circumstances surrounding the strike, the more it starts to look like one piece of a much, much larger whole.

As these things go, the main story is solid stuff. Corey Stoll makes a good addition to the cast as a Pakistan CIA station chief who's at the center of some intriguing mysteries, and Rupert Friend's Peter Quinn ably steps into the role of male lead with Brody dead and gone. By the end of episode three, the central spy storyline is spreading its tendrils everywhere, and it's hard not to wonder what's coming next.

But the problem is that most spy storylines are growers. They creep up on you, slowly enmeshing you in a world of intrigue. And that necessarily requires characters you're immediately invested and interested in. The first season of Homeland had that by the gallon, thanks to the riveting work of Damian Lewis as Brody and Danes's completely uncompromising performance as Carrie. But in that uncompromising performance were sown the seeds of the show's destruction as well. And that's why so much of the time spent with Carrie in the first three episodes of season four feels like such a chore.

The trouble with Carrie

The notion of the brilliant woman who suffers from a debilitating mental illness has become such a cliché in TV circles (mostly on shows that were quickly canceled) that it's easy to forget just how bracing this character was when she first appeared. In Danes's hands, she became a living embodiment of the raw nerve that is the war on terror. She was a constant reminder of everything the US had turned itself into to hunt out terrorists, and then people who were just sort of adjacent to terrorists, and then ... well, what then? She was self-destructive but fascinating; looking away was impossible.

But once Carrie's character became almost entirely subsumed by her relationship with Brody, the show began its spiral. The second season has become so defined by its problematic ending that it's easy to forget, but the first two-thirds of that season are filled with brilliant moments and episodes where the Carrie/Brody relationship is color that fills in their currently distrustful partnership, rather than the show's guiding force. The show might have been at its best at this point, before it bought wholly into romance and spiraled into the sun.

She was self-destructive but fascinating; looking away was impossible

And now it's just Carrie, and she's still troubled, and still bracing, and still brilliant, but it's also sort of exhausting just spending time with her. The show's treatment of her bipolar disorder has always been one of its stronger elements, but it's also something that the show can't leave behind.

Walter White could have chosen to stop cooking meth and Don Draper could choose to stop sleeping with women other than his wife. That neither does this drives much of the tension of their respective shows, reflecting the way that all of us sometimes choose the worst possible thing at the worst possible moment. But Carrie can't choose to stop being bipolar and the show, in its commitment to honor that and tell the truth of that story, has pushed her to some dark, horrible places. In particular, there's a moment in the second episode of this new season that could be a dealbreaker for many of the show's fans.

In its own way, I found that moment sort of brave and adventurous and bold for the show. It's gutsy and uncompromising, and it reminds you of just how damaged this woman is, both because of her bipolar disorder and because the government keeps using her up and wringing her out. It's like a signpost for fans of the show who might have forgotten just how horrible Carrie can be: get out before you get sucked back in.

But the show seems to have forgotten what the flipside of the character was, the brilliance of Carrie, the way that she threw herself into the worst possible choices because she needed to blow up her life to save the world. Now, instead, the show gives us reflections from other people, who tell us how she's brilliant, or tell us how she's great, or maybe even feel deeply for her in ways that are slightly unbelievable. And in those moments, it feels not just like the series has lost track of its main character but also like it's terrified that in the wake of losing Brody (and his family, it should be noted), it will have to lose a whole bunch of other things as well, so it might as well shuffle them back into the deck, even if there's no good reason to.

An otherwise solid season

This is too bad, because the rest of season four is solid television. In particular, the storyline in Pakistan — where a survivor of the wedding strike finds himself drawn steadily into political spheres he doesn't want any part of — is meaty, messy stuff, willing to engage with the inhuman horrors of modern warfare on a deep level.

Try though it might, Homeland can never quite escape its past bad decisions

In particular, the way that the show keeps throwing images of him (from directors Lesli Linka Glatter and Keith Gordon) up on screens that Carrie watches dispassionately in CIA bunkers juxtaposes the clinical nature of death raining down from the air with the actual human costs of that war. The show has always been interested in drones — both as a hot-button issue and as a metaphor for the way its characters seem unable to break free of their own programming — but season four pushes that to new places.

And showrunner Alex Gansa and his tremendous team of writers continue to have a way with a witty turn of phrase, or with complications for the non-Carrie characters. It's great to see Saul (Mandy Patinkin) and Fara (Nazanin Boniadi) again, and the show's roster of recurring characters is surprisingly deep by now. (In particular, Tracy Letts's CIA Director Lockhart increasingly feels like the only person on the show with any sense at all, which is probably a perilous position for an antagonist to be in.) The show films on location in South Africa now, making for some great visuals. And Homeland even finds a way to vaguely justify last season's stupidest decision — making Carrie pregnant with Brody's child — by leaning in to just how bad of an idea it was.

But try though it might, Homeland can never quite escape its past bad decisions, which leads it to keep making new ones. And that all comes back to the woman at the show's center. You mostly hang out with Carrie now, because you remember when she was one of TV's most exciting, interesting characters. Hell, the CIA seemingly keeps employing her because it wants her to turn the corner and become one of the best spies ever again. But the more time you spend with her, the more you realize that everything about her is right there on the surface, try though she might to break past that to her previously hidden depths.

And if you replaced "Carrie" with "Homeland" and "the CIA" with "Showtime" in the above paragraph, you'd have a pretty great summation of the series' problems at this point in time. It's still good. It's still interesting. It's still, intermittently, vital. But it's no longer brilliant, and the gap between that and what it is now is even more frustrating than if it had just become utterly terrible.

The fourth season of Homeland debuts with a two-hour premiere Sunday at 9 p.m. Eastern on Showtime.