Sequels always have a high bar to clear: living up to the promise of their predecessors as they make the continued story of their characters feel natural. While “Nymphomaniac” was designed to be released in two parts, the second half doesn’t quite live up to the first, losing a bit of momentum even as it introduces a few new wrinkles into director Lars von Trier’s complex thematic tapestry — issues of motherhood and power are added into his brew of sex, intellect and love. Although “Volume II” lacks the biting wit and energetic pacing, it brings the story of its titular sex addict to a fitting, typically abrupt close.

“Volume II” carries over the framing device from the first film, letting Joe (Charlotte Gainsbourg) lay out her extensive sexual history to Seligman (Stellan Skarsgard) after he finds her beaten in an alley. The first film ends with a cliffhanger, as Joe — who is played in some flashbacks by Stacy Martin — loses feeling in her genitals. “Volume II” follows Joe’s attempts to recapture any sort of sensation, much to the chagrin of her recurring lover, Jerome (Shia LaBeouf).

Joe and Seligman’s extended fencing match is a great comedy of contrasts, as he counters every tale of perversion with a fitting cultural allusion, much to her frustration. This winding conversation between two opposites almost seems like the two sides of von Trier engaged in conversation, which contributes to the film’s glib self-awareness. “Nymphomaniac” is constantly commenting on itself as it unfolds, with Joe directly telling the audience how to interpret a story element or informing Seligman when a digression is particularly pointless.

Gainsbourg’s performance is a lovely duet with Stacy Martin, who plays the younger Joe in flashbacks. Gainsbourg takes over the character completely in an almost imperceptible shift early in the film, and while having her step into Martin’s shoes makes narrative sense, her younger counterpart is missed. Each actress performs on a different side of the same coin, with Martin as the demure minx, shamelessly sensual and unapologetic about the trail of destruction in her wake and Gainsbourg as the ragged consequence of those actions, wearing every liaison and regret in her eyes, even as she plunges into a new pool of suitors.

Also among the returning cast is Skarsgard, a great foil, forever curious about Joe’s stories in a wholly detached manner. LaBeouf builds nicely on his work in the first film, shining in a dramatic scene where he realizes the hopelessness of Joe’s addiction. Jamie Bell joins the cast as K, one of Joe’s partners who has a penchant for ruthless S&M-style whippings. Bell is coldly commanding in his darkly funny, disturbing scenes.

As is to be expected, the energetic comedy of “Volume I” is much tougher to find in the bleaker “Volume II,” which finds Joe neglecting her newborn son in pursuit of more sex and eventually channeling her sexual aggression into a criminal career. There’s little doubt that Willem Dafoe, playing an intimidating debt collector, is one of the great victims of the severe trimming “Nymphomaniac” has received — its two-part presentation is down from a reported five-and-a-half hour cut. Unfortunately, even four hours is a little much for von Trier’s opus, and the proceedings undeniably lose steam toward the end as a few peripheral characters behave in illogical fashion only to push Joe toward the alley beating that opens “Volume I.” The film’s final moments, after von Trier issues a sweeping thesis statement about gender inequality, are similarly abrupt, making dramatic sense but being staged in a typically bizarre context.

Lars von Trier is doubtlessly one of the most challenging filmmakers working today, as evidenced by his wildly disturbing “Antichrist” — which gets a wickedly funny callout here — and by the brazenly explicit “Nymphomaniac” films. His films are never empty exploitation, however, and it could even be said that von Trier bites off more than he can chew with “Nymphomaniac,” which samples so many different themes and topics that it struggles to say anything meaningful about most of them. Nonetheless, it’s a project fully deserving of commendation, and even if it may be a bit too indulgent for its own good, “Nymphomaniac” is an entertaining dissection of sex, culture and everything in between.