Ian McKellen was surely made to play Sherlock Holmes — that lanky figure and cultivated voice are ideal for the deerstalker hat, the pipe, and the highbrow cogitating. As it happens, all three attributes are challenged in “Mr. Holmes,” Bill Condon’s film about the waning days of the world’s greatest consulting detective.

Holmes, now 93 and long retired to the countryside, irritably brushes aside the cap and the pipe as the fancies of those stories Dr. Watson used to write. More pressingly, Sherlock Holmes’ mind is fading.

As he loses his memory, he tries to put down in writing what happened in his last case, some 30 years ago. He’s forgotten the details, but he knows that something went terribly wrong.

“Mr. Holmes” is based on the novel “A Slight Trick of the Mind,” by Mitch Cullin. The elements are nearly irresistible, although the movie doesn’t unfold in the straightforward fashion of an Arthur Conan Doyle story.

As befits Holmes’ addled brain, the action periodically trips back in time. Scenes from the old case reel through Holmes’ mind, as he recalls a mystery woman (Hattie Morahan) he was hired to observe. More recently, the detective’s trip to post-WWII Japan, and his visit with a local admirer (Hiroyuki Sanada), become more significant as the film goes on.

The result is oddly similar to “Gods and Monsters,” which also paired McKellen with Condon; there’s another plain-talking housekeeper (Laura Linney here) and a naïve admirer — in this case, the housekeeper’s young son (Milo Parker), who helps Holmes with beekeeping. There’s a lot of charm in the situation, although the detective story isn’t actually that compelling by itself.

McKellen’s performance is surprisingly fussy. The actor is 76 and must play both older and younger than his real age, a ploy undermined by bad make-up and hair. He overemphasizes the comedy in the early going, as though someone got nervous about making an audience sit through what is actually a rather grim story.

But “Mr. Holmes” is sneaky, because as it rounds to its purpose, it reveals a wonderful idea at its core: Sherlock Holmes, who disdains the romance of Watson’s short stories and prefers to deal only in facts, must learn how to accept the need for fiction. Being Sherlock Holmes is a science, but telling lies is an art — an intriguing lesson rolled into this mellow tale.

“Mr. Holmes” (three stars)

Sherlock Holmes is 93 and his mind is fading — but the great detective wants to write down what happened in his last, botched detective case. Ian McKellen gives a sometimes curious performance as Holmes, but Bill Condon’s film eventually reveals an intriguing new wrinkle on the great character’s mythology.

Rating: PG, for subject matter

Showing: Alderwood Mall, Guild 45th, Oak Tree, Pacific Place