If you marry Christian Grey, you know what you signed up for.

You’re looking at long nights of being put to the test in more ways than one.

There’s always the chance of feeling helpless — bound and shackled and at his mercy — at any moment.

Not to mention the pain. Oh my, the pain.

I’m not talking about anything that transpires in the kinky billionaire’s vaunted “Red Room,” with all its whips and chains and shackles and adult toys.

I’m referring to the excruciating, mind-numbing, soul-deadening, death-by-a-thousand-paper-cuts experience of sharing your life with a joyless, petulant, possessive, deceptive, immature, lifeless stiff who can somehow manage to drain the fun out of every room he enters.

You want to talk about painful? Watch Christian Grey sitting at the piano in his obscenely expansive Aspen vacation home, murdering Paul McCartney’s “Maybe I’m Amazed.”

It’s a wonder Anastasia Steele didn’t take that moment to say, “I don’t care how much money you have and how much you look like an underwear model, for the love of God and for the sake of my own I.Q., I gotta go!”

The third and (thank the cinema gods) final chapter in the vapid, lurid, melodramatic and often unintentionally humorous S&M-sprinkled trilogy of slick trash (based on the wildly and inexplicably popular novels by E.L. James) kicks off with Ana (Dakota Johnson) and Christian (Jamie Dornan) getting married and then jetting off to Paris for their honeymoon. (Ana is astonished to learn Christian has a private jet. “This is yours?” she says. “This is OURS,” he responds. Didn’t Ana see the first two “Fifty Shades” movies, when we all learned Christian is obscenely wealthy?)

Once they’re back in Seattle, Christian occupies himself with his business — whatever that business is — and Ana returns to the little indie publishing house where she has been promoted to Fiction Editor. (She’s clearly a genius, racking up the royalties for a recently published novel by an author she discovered, and issuing directives such as, “Let’s increase the font size two points for the print edition.”)

Oooh, but Christian doesn’t like it when he tries to e-mail “Ana Grey” only to learn she hasn’t changed her name in professional circles. And he explodes when she dares to even joke about having a baby. And oh man, when she goes out for a drink with her friend without getting his blessing? He LOSES it.

What a guy.

Christian is ostensibly worried about Ana because her former and (rightfully) disgraced boss, Eric Johnson’s evil Jack Hyde (ooh, just like “Dr. Jekyll and…”) has been skulking about, plotting his revenge on Ana and Christian.

Ana’s former boss Jack Hyde (Eric Johnson) seeks revenge in “Fifty Shades Freed.” | UNIVERSAL PICTURES

But even when Ana’s safety doesn’t seem to be in question, Christian keeps his icy demeanor while maintaining the glassy glaze of a borderline sociopath.

Just to keep things moving along, “Fifty Shades Freed” often requires the main characters to act like idiots. When Ana and Christian are being tailed, Christian urges Ana to floor it and drive like a maniac, thus endangering dozens of lives. This is a wonderful opportunity for extended product placement for a certain brand of German automobile — but other options, including letting the security team handle it or, you know, calling the police, would seem to make more sense than Ana zooming in and out of traffic as if she’s a stunt driver in a Super Bowl commercial.

Of course, the newlyweds find time for slightly kinky sexual escapades, e.g., dripping and then licking ice cream off each other’s torsos in the kitchen in the middle of the night. But in the home stretch, “Fifty Shades Freed” leaves the sexy stuff behind and turns into a combo platter of a cheesy, easily solved mystery-thriller and an overwrought, daytime soap opera melodrama.

With little else to keep me interested as the story stumbled along, I did enjoy keeping track of all the trendy names of various minor supporting characters, from high-priced bodyguards Taylor and Sawyer and Prescott to the novelist Boyce Fox to the architect known as Gia Matteo to the bank manager Troy Whelan.

The story remained bland and gray (and Grey) until the merciful end, but at least the names were colorful.

Universal Pictures presents a film directed by James Foley and written by Niall Leonard, based on the novel by E.L. James. Rated R (for strong sexual content, nudity, and language). Running time: 105 minutes. Now showing at local theaters.