Truth be told, the two Oscar winners in the cast are supporting players to this “Hand that Rocks the Cradle” riff given the “Princess Bride” punchline for a title — “Inconceivable.”

It’s a Gina Gershon vs. Nicky Whelan moms-throw-down thriller, a sedentary stroll through surrogacy gone terribly, graphically, insanely wrong.

Nicolas Cage plays Brian, the doctor/husband who doesn’t believe his doctor/wife (Gershon) when she starts freaking out over this new friend/play-date stranger whom they’ve let into their lives, moved into their house and persuaded to donate the egg that will be their second child, a pregnancy she also volunteers to carry to term.

Faye Dunaway is Brian’s suspicious mother, the one who wonders about this striking single-mom blonde (Whelan) named Katie who has turned a play-date between toddler daughters into an escalating relationship where “I feel like I’m part of the family.”

Gershon is Angela, the wife/mother whose life of miscarriages and a disapproving mother in law (Dunaway) is made so much better when Katie and her little girl enter their lives.

Katie is helpful in every way. But as we’ve seen her grabbing that child and escaping what she says was an abusive marriage, escaping by stabbing the man who grabs her by the neck in the opening scene, we have to wonder about Katie.

New hair color, demonic blue contact lenses? Either she skipped out on a murder-or-self-defense trial, or there’s more to the story than what screenwriter Chloe King tells us. And King and her director move things along so slowly that we’re way ahead of her Big Reveals anyway.

Katie and Angela meet via a friend/personal trainer (Natalie Eva Marie) and get along like a house afire.

Their little girls are the same age, and play-dates carry on into sleep overs and Moms and the personal trainer finish off a bottle of wine parties.

But Katie is awfully fast taking liberties with her new status. She skinny dips in the pool. She has sex in the poolside cabana where they let her and her little girl stay.

As she takes on nanny duties, paints their house, (bedroom murals for children’s rooms are her specialty), she’s rummaging through Angela’s closets, setting her sights on…something, avoiding being photographed and dodging the mother-in-law’s pointed queries.

Pacing a picture like this too slowly is fatal, because of that whole audience-gets-ahead-of-the-movie thing. That’s on director Jonathan Baker (who has a small, overly indulged with screen-time role).

Whelan has limited effectiveness in creating doubt that Katie is not what she seems. A little too on the DeMornay nose, if you follow.

Cage gives fair value, and if it took his name to get his “Face/Off” co-star Gershon a leading lady role — Angela is the one who works at unraveling the puzzle, confirming her suspicions — then good on him for that.

Gershon makes the most of this chance, but it’s not enough.

It’s just that “Inconceivable,” despite the odd moment of tension, mystery or violence, despite attempts at delivering a decent twist or two in the third act, is too obvious to come off, too melodramatic to surprise and too slow to hold our interest.

MPAA Rating: R for some violence, sexuality, nudity and language

Cast: Gina Gershon, Nicolas Cage, Faye Dunaway, Nicky Whelan

Credits:Directed by Jonathan Baker, script by Chloe King. A Lionsgate release.

Running time: 1:46