The first “laugh” in “Monster-in-Law” is when we see two dogs humping. That is how the movie chooses to begin its 105-minute reign of generic, unimaginative comedy: canine coitus.

I am dismayed to report that it gets no better from there. This smelly dog of a film stars Jennifer Lopez, who has not played a character other than herself since 2000’s “The Cell.” This time J-Lo is named Charley and she is a carefree L.A. woman with a temp job and a multitude of side projects, her apartment cluttered with boxes and paraphernalia to indicate that she is free-spirited and unpretentious.

She chances to meet a renowned surgeon named Kevin Fields (Michael Vartan), and they fall in love. There is only one problem: Kevin’s mother, famous Oprah-like TV personality Viola Fields (Jane Fonda), who smothers and clings to her son as the last vestige of sanity in her increasingly manic life. She has just been fired from her talk show and replaced with a younger, hipper woman, which leads to Viola attacking a Britney Spears-ish guest on the air. (The show goes out live, of course, like 2 percent of TV shows in the real world and 100 percent of TV shows in movies.) Then she spends time in a treatment facility, dealing with her anger and depression. And now she learns that her son — her only son, her pride and joy — is going to marry some free-spirited, middle-class jenny-from-the-block?!

What is not clear is why, exactly, Viola opposes the marriage. The psychological reason, of course, is that she wants her son only for herself. But the movie never hints at this being even the underlying cause. If Viola has a psychological attachment to her son, no one in the film ever makes the diagnosis, not even her sassy, astute assistant Ruby (Wanda Sykes, whose acerbic commentary provides the film’s only few laughs). Instead, the film (with screenplay by first-timer Anya Kochoff) is content to tell us that Viola opposes it, and that’s all we need to know. So there.

To prevent Kevin from marrying Charly, Viola goes to extreme, ostensibly funny lengths. She feigns a nervous breakdown on the eve of Kevin’s departure for a medical convention, forcing Charly to take care of her in his absence. Viola’s stated goal is to drive Charly so crazy that she’ll call off the wedding and never look back, but it’s an awfully elaborate plan — complete with falsified prescriptions and actors posing as doctors — for a woman with such vague motivations. She’s not crazy; in fact, her scheming is quite lucid. It just doesn’t have any basis in reality.

The real problem here is that the movie never accepts that it is, at heart, a farce. Viola consciously trying to drive Charly insane, Charly discovering the plan and retaliating in kind — people, when two women use devious powers to destroy each other, it is dark comedy, not a light, frothy, soft-lensed J-Lo throwaway. But Robert Luketic, the usually competent director of such fare as “Legally Blonde” and the under-appreciated “Win a Date with Tad Hamilton!,” seems oblivious to his material’s true nature.

Unable to commit to its farcical plot, the movie thus flounders in mediocrity, with supposedly three-dimensional characters behaving one-dimensionally, and unrealistic situations being treated as though they were, in fact, possible. None of it connects because none of it makes sense — not to mention the fact that the things that are supposed to be funny simply aren’t. The gags are stale, and the performances are bland.

Jane Fonda has not appeared in a film since 1990’s “Stanley and Iris,” and I can almost see why this one brought her out of semi-retirement. In theory, it affords her the opportunity to let her hair down and be silly, the way Barbra Streisand recently did in “Meet the Fockers.” But Fonda is not the loosey-goosey type: Even if “Monster-in-Law” were written so that Viola were a full-fledged farce character, Fonda shows no signs of being up to the task. Just as she recently back-pedaled on her infamous Vietnam photographs, perhaps 35 years from now she’ll apologize for this picture, too.

D+ (1 hr., 45 min.; PG-13, scattered profanity, a bit of sexuality.)