“Dummy,” Portishead’s 1994 debut, broke a taboo that set the music world alight — white Brits unself-consciously cribbing from hip-hop. It might not seem unusual now, but back then, Eminem was still an unknown New Jack, and the Beastie Boys were the only white rappers with street cred. Assisted by jazz guitarist Adrian Utley, P-head merged phat drum sounds with torch songs and film noir soundtracks. Their creepy but sensual blend proved compelling enough to grab that year’s Mercury Prize, as well as a loyal following in the U.S.

Along with their success, Barrow and Gibbons developed a reputation for shyness with the press and seemed stupefied by their popularity. In what seemed to some like an effort to avoid the twin pillars of Barrow’s anxiety — praise and sales –the baby was tossed out with the bath water. A less sexy, more dissonant and inaccessible second LP, “Portishead,” emerged in 1997. “All Mine,” the highlight of that album, was a brassy, swinging tune reminiscent of a James Bond theme. After releasing a live album in 1998, they fell into a decade-long silence.

In the meantime, trip hop lost its juice. Tricky’s global mélange began to shed the influences of black American and lower-hemisphere music. His latest releases — like the recent single “Council Estate” — have emphasized his punk influences. Similarly, Massive Attack lost most of their members and forsook the groove. Their 2003 album “100th Window” sounded more like an answer to the post-punk atmospherics of early ’80s bands like Bauhaus.

By the turn of the millennium, it seemed as if the genre’s last hope was an album representing a return to blackness from one of its main progenitors. By turning away from dance music, Africa and the Americas, this music no longer promised cross-cultural understanding through musical cross-pollination. In the last 10 years, trip hop managed to ignore the massive influence of beat masters like Timbaland and Rodney Jerkins; younger acts like Basement Jaxx and M.I.A. picked up the torch of polyglot dance music and prevented it from being extinguished.

Portishead’s “Third” finally severs all ties from anything remotely black or cosmopolitan, aside from a couple of breakbeats. Its extravagance, repetitiveness and gloom make the album Euro and Romantic enough to sound, at times, like high camp. When they prepare to take this set of dirges on tour, Gibbons and Barrow will need a truckload of lace and black lipstick.

A monotonous breakbeat throbs. Barrow adds synths that bring to mind theremins and Farfisa organs — the stuff of 1950s horror movies — even the theme from “The Munsters.” Gibbons begins to groan, her voice ghostly and nearly operatic. “Tormented inside,” she sings. “Wounded and afraid inside my head.” This describes only the first song, “Silence.” Similar tracks recall a variety of mopey and/or industrial groups from the ’80s — Joy Division, Dead Can Dance, sometimes even Eastern European provocateurs Laibach.

Tapping the shallows of their despair, the group weaves in bummed-out folk tunes like “Hunter,” “The Rip” and “Deep Water,” which set Gibbons’ dreary delivery against Spanish guitars or mandolins with the reverb cranked to give the impression that she is singing inside an empty church or a lonely culvert. Gibbons urges herself to conquer her fear of drowning. Listeners who have not decided to drown themselves by the end of that track should grit their teeth for “Machine Gun,” whose beat sounds a lot like — guess what? More accurately, the weapon in question seems to have been re-created on an 808 drum machine by the noise-punk band Einst¨rtzende Neubauten — those guys who used to play shopping carts onstage.

Gibbons’ trembling voice used to sound bluesy and erotic –”It Could Be Sweet,” from their debut, could have been pillow talk verbatim. On the second record, she often threw in a vampy, nasal quality that cut the generally depressive tone with a touch of humor. What singer could take herself seriously while doing a Shirley Bassey impression? On “Third,” Gibbons takes herself entirely too seriously, moving between a whispery, disaffected moan and a fluttery, anxious whine. She always sounds powerless, like she’s about to burst into tears. The folky numbers, like “Hunter,” have a very listenable, austere atmosphere, but not a sensual one — Portishead has lured you and your partner to their dark Transylvanian castle, but you’ll be sleeping in separate quarters.

Gibbons’ lyrics used to suggest an assertive, liberated attitude. She closed out “Dummy” demanding a reason to be a woman; on “Elysium,” from “Portishead,” she insists, “You can’t deny how I feel/ And you can’t decide for me!” On “Third,” however, she treats her indecisiveness like a torment on the level of psychosis — a one-dimensional, even stereotypical, view of womanhood as equivocal and weak. “I’m always so unsure!” she laments on the album’s dismal closer, “Threads,” like someone about to throw herself off a bridge because she cannot afford a life coach.

The album’s final sound effect is a full minute of low, distorted and bent guitar, ringing out like a foghorn. It sounds ominous, which is cool, but its meaning lacks anything pleasurable. It only signifies, over and over, that you have just listened to a really grim pop album — in case you weren’t sure. This final sound wipes away the sorts of things that make music worth listening to and life worthwhile — playfulness, sensuality, complexity, hope and perspective. It argues that existence offers nothing but doubt and misery. Worse, Gibbons tries to prove that doubt is misery.

Wallowing certainly has its pleasures, and “Third” does a good job of encouraging those, but they ought to remain brief and private. Trip hop could never have saved the world from despair, but it did promise inclusiveness, slow dancing, sex, relaxation, pot smoking, etc., as respites from despair — all things that “Third” has laid to rest under that dearly departed genre’s tombstone.