“Unspeakable” might be a better title for M. Night Shyamalan’s followup to his 1999 smash “The Sixth Sense.” The picture mimics its predecessor in numerous ways, from its crushingly deliberate pacing and morosely blue-green color palette to a largely impassive lead performance by Bruce Willis and a Hitchcockian cameo by the writer-director himself (here as a drug dealer). It also shares with the earlier film a storyline laced with supernatural mystery and a sudden twist ending designed to take one’s breath away. Unfortunately, while “Sense” managed to maintain a sense of chilling plausibility despite its underlying outlandishness, the new picture imposes on its pulpish premise a weight it just can’t bear, and at the lugubrious speed at which the director plays the scenario out, it just grows sillier and more tiresome.

The conceit behind “Unbreakable” is that the idea of a special, nearly invulnerable being most clearly embodied in costumed comic-book superheroes might actually be a garbled reminiscence of an ancient human reality embedded in the common consciousness, and that there might really exist some chosen individuals almost impervious to harm and destined to serve as humanity’s saviors. When security guard David Dunn (Willis) emerges from a devastating train wreck without a scratch, he’s approached by Elijah Price (Samuel L. Jackson), a dealer in comic art suffering from a genetic defect which makes him subject to easy injury; Price suggests that Dunn might be such an extraordinary individual fated to protect mankind from evil–physically the mirror image, as it were, of Price himself. As the narrative proceeds, Dunn, a laconic man troubled by family problems and personal regrets, entertains Price’s initially preposterous notion and begins to wonder whether he isn’t actually one of the (non-digi)destined, as it were.

In itself the premise positing a heroic defender of humanity, though inevitably pulpish, can serve as the basis for some thought-provoking fantasy: Harlan Ellison, for example, used the notion to good effect in his 1964 “Outer Limits” episode “Demon With a Glass Hand,” and an equally intriguing variant of it appeared in his story “Paladin of the Lost Hour,” which was turned into one of the better segments of the “Twilight Zone” second series in 1985. But while Ellison’s riffs on the idea evinced a sort of mystical grandeur, Shyamalan’s relies on an unvaryingly turgid portentiousness which grows progressively infuriating as the picture unspools. It wouldn’t be cricket to reveal much of the narrative arc, but one can note that although Sherlock Holmes may be long gone, Dr. Moriarity proves to be alive and well in that dank vision of Philadelphia which is ShyamalanLand.

To be fair, one should admire the writer-director’s continuing interest in tackling philosophical, life-and-death issues in his work, even if he gives them a pop twist to make such musings palatable to a mass audience. (You can see his ambition along these lines not only in “The Sixth Sense,” but also in his lighter, little-seen first film “Wide Awake.”) One can also note that there is one element in “Unbreakable” that is a successful carry-over from “Sense,” namely the sweet, affecting relationship depicted between Dunn and his young son Joseph (Spencer Treat Clark). Shyamalan seems especially skilled in getting fine performances out of adolescents, and although Clark doesn’t have nearly the screen time that Haley Joel Osment enjoyed in “Sense,” he creates a touching, affecting portrait of a boy in awe of his father but also in obvious turmoil over his parents’ marital difficulties. On the other hand, the writer-director seems less able to fashion convincing male-female relationships: the scenes between Dunn and his wife Audrey, played decently but somewhat reticently by Robin Wright Penn, lack the intensity they ought to have. Instead the emphasis turns to the growing friendship between Dunn, who in Willis’ performance remains an almost glacial presence even when involved in physical action, and the emaciated-looking Price, whose odd appearance (his hairdo almone sets him apart) and dogged determination in the face of adversity inevitably make him a sympathetic, if somewhat strange figure.

If you value a spooky atmosphere above all else in a film and don’t mind putting up with leaden pacing and a fairly ridiculous story to get it, you might find “Unbreakable” sufficiently moody to warrant a look. But most viewers–especially those swept up by “The Sixth Sense”–are likely to find it a grave disappointment.