Atom Egoyan’s “Where the Truth Lies” may be based on a novel by Rupert Holmes, who is best known for penning “Escape (The Piña Colada Song),” but, contrary to expectations, the film is not about piña coladas and getting caught in the rain so much as pill overdoses and getting found dead in a bathtub.

From the opening shot (pan across a bathroom, ominous music, an overly-loving close-up of a naked woman drowned in a bathtub), “Where the Truth Lies” plants itself firmly in the venerable shady-side-of-showbiz genre.

This particularly sordid little tale centers on the Martin-and-Lewis-esque 1950s musical/comedy duo of Lanny Morris (Kevin Bacon) and Vince Collins (Colin Firth, distancing himself from his “Bridget Jones’s Diary” nice-guy image), who perform a yearly telethon to benefit polio victims. Behind the scenes, they indulge in plenty of sex, drugs, fights, and mob connections.

The dead girl who opens the film is Maureen O’Flaherty (Rachel Blanchard), a college student working at a Florida hotel; she brings the duo room service and stays the night. Three days later, she turns up dead in the bathtub of a New Jersey hotel.

The bulk of the film takes place 15 years later, as reporter Karen O’Connor (Alison Lohman), who idolized Morris and Collins in her youth (how amusingly Katie Holmesian), is assigned interviewing/ghostwriting duties for Collins’ sure-to-be bestselling autobiography.

She determines to get to the bottom of what really happened to Maureen that night, driven by a desire to prove herself and her obsessive interest in the pair (both motivations are hammer-to-the-head bluntly laid out in intelligence-insulting voice-over form).

In the process, she manages to sleep with Morris under an assumed identity, alienate Collins, and get plenty of standard-issue dire warnings not to stick her nose where it doesn’t belong (wonder if she’ll ignore them?).

How the girl wound up two thousand miles away and dead is an interesting little mystery, but it is ultimately the only driving plot element to the movie. Like Morris and Collins’ nightclub act, there’s enough flash and excitement that the lack of substance is not readily apparent.

There’s lots of crying, yelling, strange behavior, and kinky sex (a personal favorite for weirdness is the drugged-out scene between Karen and a girl dressed as Alice in Wonderland)—enough of the latter for the MPAA to slap the movie with an NC-17 rating. Underneath all that, however, is a simplistic whodunit (was it the loose cannon? The straight arrow? The butler?) without too many character nuances getting in the way.

Scenes with the mother of the dead girl, intended to add a more human dimension to the murder, tend toward the maudlin and ill-advised.

The central scene of the movie, which is destined for infamy (although it is actually less graphic than other sex scenes in the film), features a three-way between Morris, Collins, and Maureen in which Collins’s attraction to Morris becomes apparent and Maureen gains some blackmail material.

It seems that the central motivations for all the characters ought to hinge on that scene; however, rather than strike a deeper psychological chord or provide a plot twist, the scene ends up only tangential to the resolution of the film.

As a duo, Kevin Bacon and Colin Firth both do well with the characterizations they’re given. Firth in particular excels playing against type as a manipulative, deeply depressed loner with few redeeming characteristics. As the third main character, Alison Lohman fails to make much of an impression.

At the end, the truth of the fateful evening is revealed in a “C.S.I”-style series of flashbacks and neat summation, which leaves the audience missing the substance to which this plot had been leading.

Director/screenwriter Atom Egoyan (“The Sweet Hereafter”) deserves credit for shooting “Where The Truth Lies” prettily enough and pacing it well enough that we don’t realize until the credits roll that it was nothing more than a straightforward detective/reporter movie.

With that in mind, go ahead and see it for the mystery, see it for the sex, see it for the incredibly cheesy ’70s attire, but don’t expect the truth to lie very far beyond the surface.