[Warning: Spoilers ahead for Sunday's episode of The Walking Dead, "The Grove."]

Sunday night's "The Grove," penned by executive producer/writer Scott M. Gimple, is one of the best episodes The Walking Dead has ever done. And it got to the heart of why the drama works on so many levels other than just being a show about zombies -- and why it should be taken more seriously as a top-tier drama.

Arguably, no series on television uses children as effectively -- and as shockingly -- as The Walking Dead does when it tells its stories. And it has done this from the very start of the series when, out of nowhere, the zombie apocalypse spreads from a contagion and everybody is swept up in it -- everybody, meaning children and the elderly as well as the fleet-of-foot and fit-for-fighting adults. The decision to feature children prominently was not only bold but important, keeping the series rooted in realism, which is one of its greatest strengths beyond the most important one -- that this show is about the living rather than the dead.

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During Sunday's episode, "The Grove," Gimple wrote in not one but two shocking twists. Viewers were already familiar with two young sisters: Mika (Kyla Kenedy) and the slightly older Lizzie (Brighton Sharbino). The sisters had lost their parents and were taken under the wing of series regular Carol (Melissa McBride), whose own daughter, Sophia, was turned in the earlier days of the show and, memorably, put down as a zombie in front of Carol's eyes (one in a lengthy list of emotionally wrenching scenes that The Walking Dead has pulled off with aplomb).

Interconnected stories that run through characters as they grow are essential to making a great series. Believability and, ultimately, emotional payoff come from not only getting to know characters well, but also having them act as we might expect them to as they face adversity, change or something unexpected. That's why Carol felt it necessary to toughen up Lizzie and Mika -- she knows all too well that the meek do not survive in this new world. Truly taking care of Lizzie and Mika wasn't just about protecting them, but teaching them how to protect themselves against the "walkers." And both girls have done just that in previous episodes. But Lizzie couldn't get it right in her head that the walkers were not still somehow real people.

Mika, meanwhile, was relying too much on Lizzie's older-sister protection and Carol knew that Mika had to toughen up. "The Grove" featured a strong scene where Mika tells kill-or-be-killed Carol that she won't kill another human, but will kill walkers. Finding traces of humanity is a theme in the series, but sometimes with all the bleakness that hope has to come from within children. The Walking Dead's worldview is often unflinching -- always has been -- so it made clear from the start that other humans can be as dangerous as the walkers. When the world goes to hell and people are lost and roaming, the kindness in our hearts wants to believe that others will help, that coming together with strangers in a similar situation will make the larger group stronger. The Walking Dead has rightly chosen to subvert that in most cases -- believing that our basic instinct takes the form of the survival of the fittest. But Mika wasn't there yet (she couldn't even kill a deer, which would have been quite helpful on the hunger front).

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In Sunday's episode, Lizzie continued her dangerous inability to disassociate the walkers from humans. In one scene, Lizzie is seen "playing" chase with another young girl who just happens to be a walker. It's scary and sad at the same time, and tilts the scale to Lizzie's deepening mental schism.

Lizzie is outraged that Carol would kill her "friend," but gets past that when soon after a group of walkers nearly catch and kill Mika in the grove where they have all temporarily taken shelter. That, too, was a nicely placed scene by Gimple because it seemed as if Lizzie finally understood the walkers and what she would have to do.

She didn't. And in the first scene that rocked viewers, Carol finds that Lizzie has killed Mika with a knife -- there's blood all over her hands and Mika lies dead behind her. Lizzie believes that Mika will come back (as a walker, but in Lizzie's mind, they're not all bad). Lizzie even reveals that she was about to kill baby Judith.

And that puts in motion the next shocking scene, where Carol, tears running down her face, takes Lizzie for a walk and shoots her (as Lizzie looks at flowers, her technique to calm herself down -- and a well-constructed nod to Of Mice and Men).

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It's emotionally devastating. Killing children on television is the third rail for most shows and writers. But what made "The Grove" so powerfully exceptional was how The Walking Dead, through all of its seasons, had earned that moment. Yes, some people only watch the series for the zombie apocalypse element and they get bored when there's too much standing around and talking. But by now, most fans of this incredibly popular show have come to understand that the risk, the drama, the emotional toll that keeps the series so compelling only comes from those slower, talk-filled parts. It's where viewers get to know the characters on a deeper level by what those discussions reveal, leading to relatability, likability and the kind of connection that really reverberates back through the screen when something bad happens to them.

It's hard to explain to people who don't watch The Walking Dead just how emotional the series can be -- and how well the characters themselves express the emotion of loss, grief, anger, temporary happiness, unrelenting fear and the creeping existential depression that comes from realizing that it's probably only a matter of time before you'll die (violently).

That is so far above and beyond a mere zombie show. And when you bring children into the mix, there's a certain audaciousness in the decision. The Walking Dead has improved markedly in its handling of this issue. Carl (Chandler Riggs) used to be that annoying kid who wandered off and got himself in peril all the time. But this new world hardened him and the things he's had to do (shoot his mother before she turns into a walker, save baby Judith from that blood-drenched nightmare and hold his own as a walker-killing "adult") has transformed his role on the series and, in turn, made the series better as it fully embraces the brutality of its environment.

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That Mika was killed by her sister completely fits into the worldview that The Walking Dead has constructed. That Carol realizes Lizzie "can't be around other people" (and certainly not baby Judith) created a moral quandary that was as heavy as any you'll see on television. You don't just randomly have an adult shoot a kid dead on television. But it was believable and even felt like the right thing to do based on how hard the series has worked to earn that payoff. It would be an awful thing for Carol to have to do anyway, but knowing her backstory with Sophia and how she nurtured Lizzie and Mika to stay safe and alive added another layer of devastation to it.

The Walking Dead is often dismissed by some as a mere zombie show (genre series often get shunted aside, critically). But the series has taken far more chances than many of its top-tier competitors -- without ever getting Emmy recognition -- and Sunday's episode marked a clear point where it should start getting the respect it deserves for what it's trying to accomplish. And Gimple, who has helped shepherd the deepening resonance in the series, should be applauded for pulling off an hour of extremely difficult drama.

Email: Tim.Goodman@THR.com

Twitter: @BastardMachine