THERE are two related images that summarise why Breaking Bad, shortly returning for its final season on Netflix, has a real claim on televisual greatness.

The first comes in the show's very first episode.

Walter White, an enfeebled chemistry teacher who will shortly discover that he has terminal cancer, arrives home late from the car wash where he does shifts to make ends meet.

As he opens the front door, his terrible haircut and slug of a moustache seem to cower at the discovery of his friends and family, who are waiting to surprise him on the occasion of his 50th birthday.

The Walter White we meet is impossible not to like. But he is also pathetic.

Then there's the man that Walter White becomes. (There is a minor spoiler ahead, so read on at your own risk.)

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In that first episode, Walt decides to use his talents as a chemist to produce crystal meth in the hope of providing financial security for his family.

By Season Four, that idea has metastasized into an improvisatory, violent, and ruthless quest for power.

His wife, Skyler, now a bit better informed about his career change and frightened by a mysterious murder, is concerned that he is putting himself and their children in danger.

"You're not some hardened criminal, Walt," she says. "You're in over your head."

In fact, hardened is exactly what Walt is. And so he turns to Skyler, shaven-headed, that slug replaced by a crisp goatee, and his granite features seem to vibrate with menace, a skull, a death-mask.

"You clearly don't know who you're talking to," he says, "so let me clue you in. I am not in danger, Skyler. I am the danger. A guy opens his door and gets shot and you think that of me? No. I am the one who knocks."

Well, you wouldn't want to be a guest at his next surprise party. It is an extraordinary moment, made all the more powerful when we consider how far Walt, played by Bryan Cranston, has come.

Not for nothing did the show's creator, Vince Gilligan, pitch it as "Mr Chips to Scarface".

"Chemistry is technically the study of matter," Walter tells his class in that pilot episode.

"But I prefer to see it as the study of change." And yet it is not initially obvious, to the innocent viewer, that this is where we are going to end up.

At first, Breaking Bad played like an exceptionally dark comedy, a fish-out-of-water tale where the highest stakes were whether or not its protagonist's family would learn of his deceit.

Cranston might have been replaced by John Cleese, so broad and absurdly played were Walter's sins.

But before long Walt began to shift, and so did the tone.

I thought the show was great from the beginning, but I guess the moment that I really fell in love with it came halfway through the first series, when Walter shaves his head, reinvents himself as the genius meth cook "Heisenberg", and wreaks bloody revenge on a drug dealer who has hurt his chaotic partner in crime, Jesse.

The sequence finishes with Walt back in his car, roaring in animal satisfaction at victory in a fight he could not have imagined picking just weeks before.

His malice is impossibly seductive, and not funny at all. Wow, I thought. This show is not about what I thought it was going to be about.

Suddenly, Walter - and through him, the viewer - is being asked: is there any reason to live a moral life? If you can shrug off the guilt, why should you not do exactly as you please?

I said before that this is the man Walter "becomes", but actually, that's an incomplete account.

The genius of Breaking Bad is that we are never quite permitted to decide whether Walter has "become" evil because of circumstance and his delusional idea that he is doing it all for his family; or whether he always had the capacity for evil within him.

Walt names his alter ego for Werner Heisenberg, the originator of quantum mechanics' "uncertainty principle", which might be boiled down to the idea that when we try to measure something, we are bound to change it.

Breaking Bad's really radical step is to take that idea and apply it, in the character-obsessed form of the television serial drama, to human nature.

The only show that comes close to the same insight, Mad Men, falls down by diagnosing its hero, Don Draper, as a mysterious outsider from the off; Walter, in contrast, is on a voyage of self-discovery.

He tries to plumb the depths of his own aptitude for malice, and finds that every wicked act simply makes its successor a little more palatable.

In dying, he tries to free himself from the shackles of convention - only to discover that his personality resided in the shackles themselves.

The daring of that conceit is only viable, of course, because of the brilliance of the man tasked with expressing it.

In a cast replete with superb supporting actors - I have a particular soft spot for Bob Odenkirk's shifty lawyer Saul Goodman - Bryan Cranston is unmatched.

Indeed, his may be the standout performance of a golden era on American TV.

Anyone who remembered Cranston from the sitcom Malcolm in the Middle would be astonished, but in fact his comedian's gifts for extremity stand him in good stead here: even as the laughter gets queasier, his slapstick, jackrabbit physicality remains.

Walter is habitually hunched, his mouth always turned down in a pre-emptive grimace, the tide of panic always on the brink of washing over his defences.

And when it does - when this forgettable chemistry teacher who still, somehow, retains a hold on your sympathies draws himself up to become "the danger" - you are thrilled and terrified at the same time.

Now the series is reaching its endgame, and Walter may well be too.

Then again, his death may be too obvious a step, too straightforwardly tragic a conclusion for a show that is so ambivalent about its anti-hero's charms.

And even if he does go, it is hard to imagine that he won't take a number of people with him.

I can't wait to find out. It is a measure of Breaking Bad's cumulative, remorseless brilliance that I am also a little relieved that the end is in sight.