Despite the title not really a sequel but more a misguided prequelish follow-up to Thai martial arts star Tony Jaa’s breakthrough hit, “Ong Bak 2” proves that more can very well mean less. The opulent period piece obviously enjoyed a much larger budget than its titular predecessor (or Jaa’s intervening picture, “The Protector”), but it’s nowhere near as much fun as either of them.

Both of Jaa’s previous films have been high-flying contemporary fight-fests. They weren’t comic in the mode of Jackie Chan pictures, and had their share of hard-hitting battle scenes, but compared to the turgid seriousness of this one, they were downright lighthearted. “Ong Bak 2,” by contrast is a big period epic set in fifteenth-century Siam, and its violence is of a far nastier sort. Jaa plays Tien, the son of a royal general who’s murdered, along with his wife, by a rebel nobleman (Sarunyu Wongkrajang) who seizes power in the kingdom. Played in the early scenes by teenager Natdanai Kongthong, who’s been sent by his father to study not martial arts but dancing, Tien escapes the soldiers of the traitor, only to fall into the hands of slave traders who toss the youngster in a muddy pit with a crocodile in a Thai form of gladiatorial combat.

Happily the kid is rescued at the last minute by a horde of bandits led by Chernang (Sorapong Chatree), who takes him back to their camp and trains him to be a fighter. Kongthong morphs into Jaa, who’s become one of Chernang’s chief lieutenants, treated virtually as the bandit-king’s son. But after proving himself in combat against a bevy of fighters, Tien decides, as recollections of his unhappy past intrude in periodic flashbacks, that he must take vengeance on those who have wronged his family.

There follow a chain of action set-pieces, beginning with a single-handed assault on the camp of the slave traders (whose captives give him their thanks, of course) and then continuing with a direct attack on that vicious warlord and his henchmen. Eventually Tien winds up back at Chernang’s camp, where he must face a gauntlet of warriors before facing the man who actually killed his father and mother.

It has to be said that Jaa’s dexterity remains astonishing. He displays his skill not just at Muay Thai, which he demonstrated in the first two films, but a whole variety of chop-socky techniques, and has particular fun with elephants, hopping from back to back of a stampeding herd early on and doing tricks with another of the animals in the final fight montage. He even pauses for a lengthy dance number in mask at one point (a nod to the plot point about his early training, and also linked to the only vaguely romantic aspect of the script, involving a young girl he befriends at school who later serves the warlord’s desire for entertainment).

Unfortunately, the repetition grows tedious, and there are times when, despite all the fighting, things slow to a crawl. That dance sequence, for example, goes on far too long, and the battle that follows it is clumsily edited. But the whole film seems tonally misguided. Jaa is credited, along with Panna Rittikrai, with the direction, and on the evidence of this picture one of them (maybe both) considers himself a Thai Sergio Leone. There’s a dour solemnity to the whole enterprise that, along with the near operatic flamboyance of the style, mimics the spaghetti westerns of sixties and seventies. But Leone’s pictures had a grandeur this one never approaches, they were homages that really honored the form, but also had fun with it. This one just drags on and on. It lasts little more than an hour and a half, but feels longer than “Lawrence of Arabia.”

Technically “Ong Bak 2” is certainly an improvement on the two charmingly tacky earlier Jaa pictures. It’s shot in widescreen, and although the images sometimes look grainy, generally they’re impressive. Terdsak Janpan contributes a bombastic score that underlines everything with thundering obviousness.

It’s an old saw that a man has know know his limitations. Jaa seems to have misjudged his. He’s perfect for pictures like the first “Ong Bak” and “The Protector,” in which his pure physical prowess carries the day. But here he’s attempted too much, and not just by taking on directing duties. The story he’s trying to tell is a couple of sizes too large for him.

It would also help if Jaa could smile occasionally. It might make the audience follow his lead.