A funny thing happened on the way to ushering "ER" into semi-retirement — the medical drama surprised everyone (including NBC's powers-that-be) with a ratings resurgence.

So a long-planned three-month hiatus for the show has been canceled.

The longtime ratings juggernaut had fallen off in recent seasons, as most long-running shows do. CBS's "Without a Trace" had taken control of Thursdays at 9 p.m., although "ER" continued to run a strong second in the time slot. (It finished the 2005-06 season No. 30 in the overall ratings, which is pretty good considering "Trace" finished No. 7.)

So NBC Entertainment president Kevin Reilly came up with a plan to prop up his aging series as it entered its 13th season — he would air 13 original episodes of "ER" through the middle of December; then the new drama "The Black Donnellys" would take over the time slot, airing 13 original episodes; finally, "ER" would return in April and air its final nine episodes of the season through the end of May.

That made some degree of sense when NBC was struggling a bit in the time slot, the thinking being that 35 original episodes of the two shows would play better than 22 original episodes and a bunch of repeats of "ER."

But "Without a Trace" isn't on Thursday nights anymore. CBS moved it to Sundays and replaced it with "Shark," which isn't doing as well.

For the season to date, "ER" is up about 900,000 viewers per week and is winning the time slot among total viewers and viewers 18-49, the demographic coveted by advertisers. "Shark" is doing well enough so that CBS has already renewed it through the end of the season, although it's doing considerably less well than "Without a Trace" did. As for ABC's "Six Degrees," there's already a deathwatch looking for that show's cancellation.

For a 13-year-old show to increase its viewership is astonishing. More astonishing still is that "ER" is doing it pretty much on its own, because its lead-in, the Thursday edition of "Deal or No Deal," is getting murdered in the ratings up against "CSI" and "Grey's Anatomy."

So ... that plan to send "ER" on a three-month hiatus is out the window. We'll get some reruns of the medical drama (including tonight at 9 p.m.), but, at this point, NBC is actually talking about ordering a couple more episodes of "ER" for a total of 24 this season.

"The Black Donnellys" will still get on the air at some point. NBC certainly has other shows that are struggling, and it will need to fill out Sundays come January when the NFL season ends.

As for "ER," there's no end in sight.

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E-mail: pierce@desnews.com