There’s much heart and humor in “The Best Man Holiday,” a sequel nearly 15 years in the making. The film’s sudden and extreme shifts in tone, however, may leave audiences shell-shocked. It jumps from laughter to tears in the blink of a false eyelash.

A Christmas season-set romantic comedy spiked with drama and tears, this follow-up to “The Best Man” reunites the college friends seen in that 1999 film, catching up with their varied lives and gathering them together again in one very large house.

Holiday cheer and the supposedly joyful experience of seeing old friends can’t stop old tensions, jealousies and rivalries from resurfacing. There’s even a girl fight or, at this point in the characters’ lives, a lady fight.

The script pours melodrama on thickly. So, too, predictable plot turns. But it’s still fun seeing Harper, Lance, Mia, Shelby, Murch, Robyn, Jordan, Quentin and Candy together again. And the movie’s ensemble cast has easy rapport.

With so many returning characters and new characters as well, including children, it takes a seating chart to keep up. For the totally uninitiated, there’s a quick refresher course at the start of the film.

At first, the major storyline in “The Best Man Holiday” seems to be Harper and his failing career as a novelist. Taye Diggs returns as Harper, giving a straight man performance that helps negate some of the film’s excess melodrama.

Harper is keeping a secret from his pregnant wife, Robyn (Sanaa Lathan). New York University just laid him off. On top of that bad news, Harper’s literary agent told him that his latest novel is unsellable. With a baby on the way, Harper doesn’t know where the money is coming from. It’s a soap-opera situation.

Harper’s tense relationship with Lance, now an NFL football star, is another storyline. When Harper and Robyn first get to Lance and his wife Mia’s mansion, Lance, because of a still uncomfortable event that happened in “The Best Man,” is not happy to see to his old friend. Co-starring as Lance, Morris Chestnut is more than wooden. He’s stone-faced.

The story’s female characters include Monica Calhoun’s Mia, the settled, thoughtful person who instigated this Christmas reunion. At the opposite end of the sensible spectrum, there’s Melissa De Sousa as Shelby, outsized troublemaker and current reality TV star.

The guys also include Terrence Howard’s ready-for-anything Quentin and Harold Perrineau’s panicky Murch. They provide comic relief. Murch, a respected educator whose proper academic world is suddenly threatened, has awkward moments but Howard easily steals the film, getting most of the laughs as the crazy, in a laid-back way, Quentin.

Fans of the first film likely will enjoy revisiting these familiar characters in “The Best Man Holiday.” But the new “Best Man” never figures out whether it’s a comedy, drama or farce.

And it doesn’t quite know what it’s about, or who it’s about, never mind what it wants to say. Instead of inventive Christmas ornaments on its tree, it hangs clichés. This Christmas visit is a mixed blessing.