A great villain can usurp the fortitude of a great hero. Look at 'The Dark Knight' — is Batman even in that movie? Heath Ledger's Joker was an instantly iconic performance that overshadowed the small effort put into complicating the Bruce Wayne character. Worth the sacrifice, but Christopher Nolan's Bat-sequel is forever flawed.

Creators Bryan Konietzko and Michael Dante DiMartino actively work to avoid having a villain problem in 'Legend of Korra.' Book 1 tethered Amon to its evolutionary introductions; As Korra learned about her new responsibilities as Avatar, the audience witnessed, through the motives of a villain, how the world shifted since 'Avatar: The Last Airbender' days. Book 2 ran into issues integrating a new villain who could both take on Korra and not steal the show. The result was the baseless Unalaq, kept understated while the season piled on the plot to compensate. Book 3 solves the problem by running Korra and Zaheer, her formidable, enigmatic opponent, in parallel lines. Even after breaking out of jail in “A Breath of Fresh Air” and causing havoc across the land, the Avatar and Book 3's Big Bad never meet. Until now.

Even when “The Terror Within” picks up, Zaheer is just a celestial object in Korra's mind. He exists ... somewhere ... outside the safeguard of Zaofu. After an eruptive argument between Lin and Sue, Team Avatar is at peace (and enjoying more kale!). Opal is to go off to train with the Airbenders. Korra, Asami, Mako, and a longing Bolin will go off to find more Airbenders — with or without the help of Varrick's Airbender Tracker ("You have to airbend into it!"). What we could mistake for naivete is the comfort that comes from a long string of wins. You see it in real life: After an attack or a tragic burst of violence, there's a moment of universal nervousness that mellows out into selective memory. Team Avatar currently in “Why do we even bother with TSA scanning at the airport?” mode.

And then, like disaster movie pseudo-science, Zaheer reveals himself as a Texas-sized asteroid plummeting towards Earth. The setup even feels like a Roland Emmerich-style blockbuster. Opal takes off in an airship, the team says their goodnights, Zaofu closes up its metal petals for protection, and then, out of the darkness, Red Lotus descends upon the unsuspecting crowd. It's a delayed explosion. At first, director Colin Heck plays Zaheer's kidnapping scheme like a scene out of Satsuo Yamamoto's 'Ninja, a Band of Assassins' — the stealthiest of ninja movies. When Bolin catches them in the act of snatching up Korra, “The Terror Within” has its '2012' moment, the Red Lotus raining down hellfire as Team Avatar strikes back with their own elemental cacophony.

We've seen battles involving hoards of benders going at it like it's the Crusades, but Red Lotus versus Zaofu's armed protectors is the series' splashiest “war” scene. All four bending powers out full force is an animation buff's dream come true: Curled metal encasements trounced by lava-bending, cables slinging into Zaheer's quartet, cutting off his airbendering diversions, Ming-Hua and P'li back-to-back like an evil version of Will Smith and Martin Lawrence in 'Bad Boys' — Heck keeps every gear spinning. A “bullet time” moment where P'li spots an incoming attack is Studio Mir's artistry at its finest, never cheapening the sensation with blur effects. It's pristine.

The operation to pluck Korra out of the Red Lotus' clutches is clever, practical, and tense: Lin and Sue swoop down from the top of Zaofu's roof, Bolin hits Combustion Girl in the noggin', cutting off her powers, and the earthbending sisters Spider-Man out of harm's way with an unconscious Korra in tow. More riveting than the plan is the imperfect execution; Zaheer cuts Sue and Lin off at the pass. The magic of 'Korra' is how an action-heavy sequence can wander back to character. Sue uses her Cirque du Soleil abilities to swing Lin and Korra to safety while sending two daggers into Zaheer's glider. The “Old Wounds” are healed. These are polar opposite sisters fighting in tandem.

The major set piece winds up so immersive, it's a little shocking how much more writer Joshua Hamilton is able to stuff into the backend of the episode. The Red Lotus' intrusion and escape raise eyebrows in Zaofu. Someone must have helped them in. Who's the mole? Like even the best episodes of '24,' it's obvious to us, but totally off Team Avatar's radar. It takes a random interruption from Varrick — who's even better this season as a quippy Ben Linus-type — and some keen detective work from Mako (after receiving the short end of the stick in the past few episodes) to pin down Aiwei, Sue's human lie detector, as the culprit.

It's a total “Hail Hydra,” crumbling, perfect world Sue created for herself. Team Avatar discovers a Secret Tunnel in Aiwei's basement, all the proof they need to tie him to the crime. The traitorous earthbender wastes little time in fleeing the scene, igniting the second, non-bending explosion in 'Korra' history (and with double the oomph than the blast that killed Amon and Tarrlok) to keep the Avatar at bay. The encounter leaves Korra with more questions than answers: Even after meeting Zaheer face-to-face, she still doesn't know who he is, where he came from, why he wants to kidnap her, or what all this violence is building towards. All she has is Aiwei's sinister smile and his forboding words: “You have no idea what's coming for you, Avatar...” She really doesn't.

As if to respond to criticisms that Lin and Sue's make-up session wrapped up a 30-year feud with too easily, “The Terror Within” closes with the rebellious, younger half-sister sliding back into her own tricks — if only for Korra's sake. Lin's still learning a thing or two about familial democracy. When Korra insists on chasing down Zaheer, she tells her surrogate teen to sit tight, butting heads with Sue and taking comfort in everyone trusting her instincts. Psych. Sue sends Korra off to avenge her, a decision Lin will probably (hopefully?) take in stride when she wakes up in the morning. Decisive action is inevitable — especially with only a handful of episodes left.

As riveting as it is to see the Joker screw with Batman, chaos burst through existence like a cannonball, Zaheer's righteous plotting is exactly what the series needed. In Book 2, we saw a literal nega-Avatar, a fight that became all brawn even with spirituality at the center of the conversation. With the Red Lotus, we see an inversion of Team Avatar that will demand Korra's consideration. There could be an entire series dedicated to the Zaheer Crew's exploits, plodding along a journey like this ATLA-inspired season. But in 'Korra''s heroic context, they're villains. Book 3 will likely push deeper into moral grey zone, challenging Korra with a problem bending can't fix. The Joker can be punched in the face. A complicated methodology like Zaheer's requires something more.