There may be some wayward irony in the fact that the last Peter Sellers movie looks as though it were directed by Inspector Clouseau. That scarcely constitutes adequate compensation, however, for enduring "The Fiendish Plot of Dr. Fu Manchu," an indefensibly inept comedy opening today at area theaters.

It might seem that any excuse to see the late Mr. Sellers make one final raid on his disguise kit would be welcome. Even so, "fu" is no excuse. It is hard to name another good actor who ever made so many bad movies as Sellers, a comedian of great gifts but ferociously faultt judgment. "Manchu" will take its rightful place alongside such colossally ill-advised washouts as "Tell Me Where It Hurts," "The Bobo" and "The Prisoner of Zenda."

Fortunately, there are a number of cherishable, extraordinary Sellers films which will survive for years and perpetuate his reputation as the man of a thousand farces. You have to strain every synapse of your brain, however, to see anything of his immortal Clouseau or inspired Strangelove or cunningly funny Quilty in the roles Sellers plays in "Manchu."

These include the title character, a diabolical fiend dreamed up by Sax Rohmer for series of novels the movies have tapped before, and also the part of Manchu's prissy nemesis Nayland Smith, a drudge from Scotland Yard. The time is hardly ripe for reviving this tiresome twosome any more than a planned exhumation of Charlie Chan seems a scintillating prospect. Someday the people who make movies will start having ideas of their own again. Perchance.

The writers, Jim Moloney and Rudy Dochtermann, and the director. Piers Haggard, never devised anything so concrete as an attitude or anything so rudimentary as a rationale for the picture, which bobs dopily about between satire and burlesque but never goes far enought in either direction. You do feel sorry for Sellers in that he is given no comic support whatsoever, but you also get the impression that the director was so terrified of him that he never dared offer a peep of counsel or doubt about what was being filmed.

Sellers looks weary and annoyed throughout the picture. His capers never attain the level of enlightened lunacy of which he was capable. If anything, the way he plays Nayland Smith suggests only a slightly less catatonic rendition of the Chauncey Gardiner route with which he almost won an Oscar in "Being There."

The film opens with a mildy promising pre-credit sequence in which Fu's demonic organic recital segues from that old despot's standard "Toccatta and Fugue" to a couple choruses of "Happy Birthday to Fu," as the reclusive megalomaniac turns 168. Time comes for the ceremonial drinking of his magical youth elixir but the dram is dribbled away by a lackey -- Burt Kwouk, the invaluable Cato of the "Pink Panther" films, in a cameo appearance ("Your face is familiar," Sellers says to him).

Then the search is on for the diamonds that will be distilled into a new bottle of youth juice. Unfortunately, the plot device requires that Sellers as Fu be enfeebled and faint through much of the film (he is occasionally reenergized with electrical shocks); in addition. Sellers' portrays Smith is a puttering dotterer. This flaccid tandem becomes spiritually enervating within about 15 minutes.

Sid Caesar is reduced to the flunky role of an American FBI agent who stands around summoning up blank reactions and thanks to the writers' least appealing and most appalling gambit, also repeatedly refers to the Chinese with the racist epithet of "chink." You wouldn't want to be in the same movie house in which a bit like that gets chuckles.

Director Haggard (nephew of H. Ryder Haggard, for what that's worth is incompetent almost beyond belief, lingering over tedious expositional trivia and undercutting what few jokes the script contains. He shows all the comedic instincts and funsy impulses of Leonid Brezhnev. The film was crudely tossed together by among others. Hugh M. Hefner and his Playboy Productions Inc., which so far has logged one of the most inauspicious records in movie history.

"You can't fool me with a cheap cinematic trick!" Sellers barks to his suddenly appearing henchmen at one point. It's a rejoiner the audience might wish to echo.