It drew a 47 percent share of viewers watching TV at that hour, which are Super Bowl numbers here in the States. The last American Idol finale, by comparison, claimed an audience share in the low 30s. Doctor Who, in England, is likely to end the year as that nation's most-viewed entertainment program.

Over here, the question may be less "Doctor Who"? -- and more "Doctor Why"?

In the U.K., Doctor Who was launched in 1963, in the aftermath of the John F. Kennedy assassination. Its first big villains were slow-talking robots named The Daleks, and the Doctor was played by William Hartnell, who portrayed a Time Lord capable of traveling through time and space. He also had the gift of regeneration, with which he could, at the moment of death, metamorphose into someone who looked and sounded completely different.

Time Lords, according to that early mythology, could regenerate a dozen times. When Doctor Who was launched, that arbitrary figure may as well have been 100 -- though it came in handy in 1966, when Hartnell was replaced by Patrick Troughton, becoming the "second" Doctor Who.

But 45 years later, with the show still running, current series star David Tennant is, by most counts, the 10th Doctor, and tonight's finale leads the way for him, too, to exit the show, after a series of quarterly specials in 2009. After he's replaced, the last Time Lord should have one life left.

Though, already, he's given life to such spinoffs as Torchwood and The Sarah Jane Adventures, and could well spawn another with lovely Georgia Moffett as the doctor's cloned daughter.

What's so much fun about this fourth-season finale is how it presents itself as a big deal to fans. Billie Piper, who played the Doctor's assistant Rose before moving on to Secret Diary of a Call Girl, is back. So is another former assistant, Freema Agyeman's Dr. Martha Jones. And John Barrowman's Captain Jack Harkness, now starring in Torchwood. And Elisabeth Sladen, now a grown-up former assistant to a former doctor, starring in The Sarah Jane Adventures.

What fun. And yet, series writer-producer Russell T. Davies, who has run and revitalized the show for four seasons now, has been brilliant this year at mixing the silly sci-fi fun with actual scares and dramatic scenes. His plots, and his characters, have depth.

And in a show with time at its center, he's very mindful and respectful of history: The big villains in the finale are the Daleks and their evil creator, Davros, updated versions of the Big Bads from the time of the LBJ administration.

When almost half a country's TV sets in use are tuned to the same thing, you know some chord is being struck. Watch Doctor Who tonight, and try to figure out why...