This is a story that may finally prove – per screenwriter William Goldman’s indelible line – that nobody knows anything in Hollywood.

Or it may, in fact, prove that everybody knows something – they’re just not always sure they know what they know until they know it.

What this definitely establishes is that television – often a fool’s paradise of predictions, guesswork, spin, hype and outright balderdash – very much remains a place of surprise and even mystery.

This is a story, in other words, about “Grey’s Anatomy” and “ER,” which both air on Thursday nights, the former at 9 on ABC, the latter at 10 on NBC.

One is the great ascendant hit of the year, the TV monster of the moment. The other is an old warhorse. Presumed to be tired and listless, this old nag was supposed to be much closer to the grave than the cradle.

“Grey’s” has been seen by an average of 24 million viewers this season, and is far and away TV’s dominant scripted show. Before this, ABC hadn’t had a No. 1 hit on Thursday night in nearly 30 years, when “Laverne & Shirley” briefly nested there during the 1979 season.

And the old warhorse? “ER” was TV’s big kahuna in the 1995, ’96 and ’98 seasons and has been a top-10 stalwart for almost each of the past 13 seasons. In terms of pure commercial potency, this has been one of the stellar hits in network history – and remained so until senescence struck. With all original cast members gone, ratings way down in 2005 and a prevailing sense (even at NBC) that the show’s creative arteries were irreversibly sclerotic, this was expected to be the last or next-to-last season.

Now, take another look: So far this season, “ER” is NBC’s most-viewed scripted series (averaging around 15 million viewers) and virtually tied with the network’s hot Monday newcomer “Heroes” among younger viewers (both get around 6.5 percent of adults between the ages of 18 and 49 in their respective time periods).

“ER,” once again, is winning the 10 p.m. time period and is even acting like a young whippersnapper in the process. Sclerosis be damned.

Besides the fact that, until now, aging hits almost never – make that never – reversed steep downward spirals, why is this so unusual?

There are a couple of reasons. First, counterintuitive though this may seem, “ER” is actually the better of the two Thursday hospital shows. Gone are last season’s histrionics (and violent nuttiness). With volume turned down, the show has gotten back to doing what it always did best – telling the stories of average people in extreme peril. The recent “Ames v. Kovac” episode with guest star Forest Whitaker was superb and went someplace (the courtroom) where “ER” has rarely ventured.

And while the drama seems richer and sturdier than in recent seasons, the show also has shrewdly added its own stubble-faced counterpart to “Grey’s” Dr. McDreamy, played by Patrick Dempsey – John Stamos as Dr. Tony Gates. Stamos is nearly the same vintage (43) as Dempsey (40), and both were late-’80s teen icons.

The other likely reason for “ER’s” recovery is equally counterintuitive (until you actually stop to think about it). “Grey’s Anatomy” is probably the key reason that “ER” is back on top. Of the resurgence, “ER’s” current show-runner and executive producer, David Zabel, says bluntly: “No, I didn’t anticipate this. I was afraid that what the audience would feel is that if they saw one (hospital) show at 9, then they might not want to see another one at 10. I’m happy to say I was wrong.”

He was wrong, but wrong for an understandable reason. What’s happening is one of the older (and odder) phenoms in television. It’s called “drafting” an audience from one show to another. In plain English, this means that if a program (“Grey’s Anatomy”) on one network is a huge hit, then another that follows on a competing network literally gets viewers in its “draft.”

The irony is that “ER” appears to be as much a beneficiary as the ABC show that should be getting the direct assist, “Six Degrees,” which follows “Grey’s” at 10.

Networks don’t particularly like the draft effect for that reason, but because the TV audience is a rather fickle and uncontrollable beast, they can’t entirely eliminate it.

Over the years, the networks got smart. Most of the heavy-hitter hour dramas were slotted at 10. Sitcoms became less prone to draft because they were designed as lead-outs or lead-ins to their companion shows. The advent of the 500-channel universe made drafting even more infrequent – or at least irrelevant.

Jeff Bader, executive vice president of ABC Entertainment and the network’s boss in charge of prime-time planning and scheduling, says, “I do think it’s drafting a little bit of ‘Grey’s Anatomy,’ but you see that in a lot of time periods. If you have a huge show on one network and a popular one on another, you will see people draft over to it.”

Not that Bader has anything to worry about. He was the architect of the decade’s most successful scheduling change so far.

So why put “Grey’s” on Thursday in the first place? To follow the money. Commercials can be sold at a premium because of the looming weekend shopping days and because there are so many more available viewers.

Nevertheless, there is studied pretentiousness to “Grey’s” these nights. This feels like a show that is Important, or – more to the point – knows that it’s Important. There was a casual serendipity to “Grey’s” last season. By contrast, “ER” now feels more grounded and, surprisingly, often more intelligent.

Credit for the recharged “ER” goes to Zabel, who has been with “ER” for five seasons. He secured Stamos for a couple of episodes last season, then signed him full time for this one. Says Zabel of Stamos’ character, new intern Tony Gates, “he has a great sense of humor and spark and energy that I felt this show needed.”

Along with “ER” stalwarts Goran Visnjic (Luka Kovac), Maura Tierney (Abby Lockhart) and Mekhi Phifer (Gregory Pratt), the show has added new faces, notably veteran actor J.P. Manoux as obnoxious motormouth Dr. Dustin Crenshaw, who may be the ideal choice for fans who actually pine for Paul McCrane’s Dr. Robert Romano.

“What’s happening this season,” Zabel says, “is that we’re getting new viewers who are young enough that they’re discovering ‘ER’ as if it’s a new show. They don’t have any preconceptions.”

They do now. They’re mostly good ones, too.