There are two main obstacles to enjoying "The Last Witch Hunter."

One is your ability to buy Vin Diesel as an immortal slayer of evildoers plying his trade in today's Manhattan. You also have to swallow a by-the-numbers plot buried under an avalanche of fast-and-furious but underwhelming CGI effects.

We first meet the usually affable Diesel as a 14th-century knight named Kaulder, with braided beard, hunting down the Witch Queen — seriously ugly! — who has unleashed the Black Death to destroy the human race.

As they battle, she curses him with immortality. So hundreds of years later he's still pursuing witches, living in a fancy Manhattan apartment, but minus the beard and weird hairdo.

Witches now coexist with humans in an uneasy truce, though the really bad ones are imprisoned. Kaulder keeps tabs on them, smacking down the worst ones and working for a dynasty of Catholic priests known as the Dolan (I'll leave it to you to decide what that's supposed to mean). When the current Dolan (Michael Caine, coasting) is put under a curse, Kaulder works with his successor (Elijah Wood) to rescue him — and, of course, the rest of humanity.

This kind of quest demands plentiful special effects, and that's what we get — mostly images of fire, disgusting bugs and creepy trees with slithering limbs. Along the way Kaulder acquires a lively sidekick (Rose Leslie, who gives the movie a boost), a beneficent witch with special powers who runs a tavern serving various inebriating and magical potions.

Director Breck Eisner (son of onetime Hollywood kingpin Michael Eisner) seems to be aiming squarely at the fanboys, since many of the effects suggest role-playing video games with magical and medieval themes. The dialogue is heavy on menacing (and other) platitudes, with very occasional, and refreshing, stabs at humor.

I don't mind Diesel at all, especially when he's winking at his muscle-bound, macho image. But that's at a minimum in "The Last Witch Hunter," which instead tries to round out the character with soft-focus flashbacks to his long-lost family life.

There's enough goodwill for Diesel that he'll survive this mess, which is clearly intended to kick off a franchise. So what are the chances of a "Witch Hunter II"? I'd say nada.