It’s an unwritten law of Hollywood. Hit movies beget sequels.

And depending on the storytelling potential in the original films and the talent of the filmmakers, sequels can be even better than the originals. But that’s a rarity.

“Pitch Perfect 2,” an energetic but underachieving follow-up to 2012’s fun, lively musical comedy about an all-female college a cappella singing group, recycles the original film’s successful formula: The young ladies who sing with the Barden Bellas, including Anna Kendrick as serious music student Beca and Rebel Wilson as Fat Amy, the story’s principal comedienne; more singing competitions; some gross-out humor; and LSU in the role of Barden University.

“Pitch Perfect” sometimes goes off key, but its performance in total sustains clever dialogue and a stronger story than “Pitch Perfect 2.” The sequel plays like a collection of sitcom sequences, most less entertaining than the Barden Bellas’ earlier hijinks. This second time around, the freshness that helped make the original movie and its amusing, ethnic rainbow of Barden Bellas charming is missing.

“Pitch Perfect 2” opens as the Barden Bellas, now seniors and three-time national a cappella singing champions, experience a competition disaster. The group gets banned from all future competition in the United States.

With their opportunities in the U.S. down to zero, the Bellas turn to international possibilities. They begin rehearsing for the world championship of a cappella competition in Denmark.

The script too conveniently places the Barden Bellas and their chief opponents for the international title, a super-precise, militaristic coed German group called Das Sound Machine, in a few scenes together before the championship contest in Copenhagen.

Birgitte Hjort Sorensen co-stars as Das Sound Machine’s condescending, fearless leader, Kommissar. During Beca and the Barden Bellas’ rocky first encounter with Kommissar, the comparatively soft Americans are defenseless against the superior musical forces that power Das Sound Machine’s Germanic onslaught. Even Beca’s verbal comebacks to Kommissar come off as reverse putdowns.

Before Copenhagen, Das Sound Machine, The Bellas and other singing groups battle during a clandestine singing competition near the Barden campus. The screenwriters copied the scene straight from the LSU-, Huey P. Long Pool-filmed competition scene in “Pitch Perfect.” Another example of the movie’s lack of fresh ideas.

Fortunately for the Bellas, they get reinforcements. Actually, one reinforcement. Hailee Steinfeld (“True Grit,” “Ender’s Game”) brings a fresh character and performance to the film as Emily. A new member of the Bellas, Emily’s mom (Katey Sagal in some flat cameos) sang with the group when she attended Barden. A sliver of plot and some sweet harmonies develop between Emily and Beca.

“Pitch Perfect 2” will be singing to the choir this weekend when millions of girls flock to theaters to see the new adventures of Beca, Fat Amy and their Barden Bellas sisters. Affection for and familiarity with the characters is the only prerequisite.