I was in a darkened theater, nestled beside a person I loved. The movie depicted two tangle-limbed lovers reveling in the throes of passion.

But let me clarify. That loved one beside me was my grandmother. And the on-screen paramours were somebody else’s – grandparents, that is.

Smack! A smooth, firm hand covered my eyes, blinding me to the illicit imagery. Grandma didn’t realize, though, the futility of her gesture, that seniors’ most shocking side was yet to be revealed … and that she would be doing the revealing.

“Grandma!” I protested. She grudgingly loosened her grip, despairing, “If I had known it would be this sort of thing … Oh, I could kill myself!”

We were watching “Innocence,” an indie pic about septuagenarians reunited 50 years after a torrid affair. I thought that Grandma might enjoy seeing seniors on screen. But neither of us had counted on seeing so much of those seniors. Normally so gentle and mild, my grandmother was beside herself, literally farblondzhet. The lady plotzed. And hey, who are we kidding? I was more than a trifle uncomfortable with watching senior citizen soft-core porn alongside Grandma. How did we go so terribly, terribly wrong?

We tried to block out the wrinkly wrangling and instead discussed the characters as we walked to the car. Grandma opposed the married woman’s decision to shack up with her old (and we’re talking old) flame. If people make commitments, Grandma reasoned, they have to face the consequences.

But Grandma reversed her righteousness quicker than George W. Bush could proclaim, “Polluters unite!” Peering through her locked car windows, we spied Grandma’s keys splayed across the driver’s seat, as tantalizing and remote as Tahiti, free textbooks or a Spice Girls reunion.

“Let’s not tell Grandpa,” she urged.

Grandma then began hatching a scheme in which she would call my mother, who would drive over with a spare set of car keys as well as take me home, so that Grandma could speed directly to her place.

“I think I can make it before dinner,” she fretted. “But with rush hour traffic, it’s going to be tight.”

Flabbergasted puts it mildly. On a dime, Grandma had turned from High Priestess of Responsibility to Queen of Conspiracy. And this wasn’t even high-level espionage she was proposing – her stunt rivaled double dining on chicken and StoveTop, replacing the Brady’s family portrait or gambling for the Elvis statue on “Saved by the Bell.” What would be next, climbing through Doogie’s window, Vinnie Delpino-style? Who was this impulsive woman before me? And why did I feel closer to her than ever before?

My brother and I once spent hours scrubbing the kitchen after an all-out food fight. Like the traces of chocolate sauce left clinging to the cabinets, the feverish “please don’t let me get caught” feeling hasn’t quite washed away – it still creeps up on me when I’m stuck in a jam. Suddenly, I recognized it in Grandma. Call it a survival instinct, a somewhat immoral but unmistakably human impulse to save her skin – regardless of its laugh lines or liver spots.

Months later in Florida, I made my way from poolside to deli, chatting, schmoozing, kibbitzing, kvetching. My company was the aged and the experience was invaluable. According to the Federal Interagency Forum on Aging-Related Statistics, 35 million Americans are age 65 or older. And stop the presses – those old folks are people!

That unsettling afternoon, I started to believe in seniors’ humanity. Although our friends at Viagra have been saying for years that the blue-haired boast libidos, it took “Innocence” to prove it. And in a moment of weakness, Grandma transformed from an institution into a girl just like me.

By the late 2040’s, I’ll be a senior myself. I only hope that I’ll be as cool as Gram – and that when Gramps and I get it on, we keep our porn private.