Chemistry, Walter White said in the first episode of "Breaking Bad," is the study of change. And with Sunday's start of the show's fifth and final season, everything has changed for "Breaking Bad" except its brilliant mix of character-driven drama, science-inspired set pieces and cave-dark comedy.

Some of the most volatile characters this time around weren't even in that first episode. And the ones that were have turned into different people: Walt (Bryan Cranston) has nearly completed his long-promised transformation from Mr. Chips to Scarface. Jesse Pinkman (Aaron Paul) is fiercely capable and utterly in the thrall of Walt, the cancer-stricken chemistry teacher he once mocked. The cocky Hank (Dean Norris) has fallen several rungs -- but is smarter and more of a threat than he's ever been. And Skyler (Anna Gunn), Walt's wife, has transformed from his partner-in-crime to a woman in far over her head.

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The challenge for "Breaking Bad" has always been topping itself, never more so than now: Season 4's finale has already gone down as one of television's best.

In that episode, Walt killed graceful drug lord Gustavo Fring in a way that literally and figuratively ripped off the false face he showed the world. Where do you go after introducing and killing Fring, a villain to rival Hannibal Lecter or Darth Vader? (He arguably surpassed them, since their creators went back and gave them cloying origin stories.)

"Breaking Bad" has never had a problem heightening drama. If it did, it couldn't have surpassed its second-season ending, in which Walt's selfishness indirectly crashed a 737.

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The first two episodes of season 5, unsurprisingly, find White looking back, hoping he's tied up every loose end in the scheme to kill Gus, which involved a bomb-rigged wheelchair and poisoning a small child.

But loose ends of course remain. And unravel. And once again, it's science to the rescue.

One of the delirious thrills of the show is how creator Vince Gilligan plays people off one another like unstable elements -- but gives actual elements space to interact as well. Often disastrously. Did you know before "Breaking Bad" what Mylar balloons do to power lines? Me neither. 

Like high school chemistry, "Breaking Bad" looks dusty and even depressing on the surface. Its Albuquerque setting has all the sand and sun but none of the cool waves of beachy dramas on USA or the networks.

But also like high school chemistry, "Breaking Bad" is most rewarding for those who pay the closest attention. It's a show where an aside like Walt's fondness for Denny's -- or the cute way Skyler marked his 50th birthday breakfast in the first episode -- get referenced again in an utterly unexpected way.

Try looking for plot holes on "Breaking Bad." By the time you think to say, "But what about the (blank)," the blank will have metastasized into some nauseatingly malicious new threat.

So it goes in the first two episodes of season 5. And like that rare student who wonders what happens if you mix this with that, you'll imagine endless incendiary interactions as you try to guess Walt's final fate: Does Skyler turn him in? Does Jesse realize how much Walt is manipulating him? Does his "criminal lawyer" (Bob Odenkirk's Saul Goodman) sell him out? And what of Mike, Gus's ice-cold enforcer? And the IRS? And the DEA? And is whatever kept the Mexican cartel from killing Gus last season going to come back to get Walt?

There are a lot of balls in the air, floating like Mylar balloons. And a lot of power lines.

The fifth and final season of "Breaking Bad" airs Sunday at 10/9c on AMC. The first eight episodes of the season will air this summer, and the last eight next year.