Pay no attention to the title. “Underworld: Evolution” has nothing to do with Hades or Darwin.

As a film about hellish vampires and werewolves, “Evolution” falls so short of being good that one might say it makes the Buffy musical episode seem avant-garde. Then again, as a descendant of 2003’s underwhelming “Underworld,” this newest installment could be cited as evidence for the idea that like breeds like, with variations sometimes beneficial, sometimes quite the reverse.

Well-suited — in head-to-toe black rubber — is Kate Beckinsale, who returns as the butt-kicking/slaying vampiress Selene. Defender of her own race against the Lycan werewolves, her responsibility in the sequel is to protect herself and her half-human, half-werewolf, half-clothed lover, Michael, played by Scott Speedman (formely “Ben” on The WB’s “Felicity”). Judging from her overacted facial expressions, Selene is confused about her long past, her uncertain future and her present place in a chaotic world. No, it does not take place in high school.

The film opens with a lengthy flashback to the Dark(er) Age of 1202 A.D., where the first vampires and werewolves begin to engage in a war that will last for centuries. Wreaking havoc and betrayal, essentially making up a history to justify their future actions, they intimidate, decapitate and salivate in the midst of rancid gore. Some scenes are so repugnant that sitting through them all but requires an antacid.

Next come more flashbacks, this time to footage of the original film (cheaters!) intended to clarify the murky plot. Such clarity, however, never arrives. This is excusable, though, because in all honesty, no one really cares if the plot makes sense. Action — murderous, gut-squishing, head-popping action — is what people who enjoy films like this want, and what they ultimately get from “Underworld: Evolution.” All that matters is that there are enough bad guys and good guys for massive slaughter.

This time, the bad guy takes on one of the most maligned forms imaginable — a slimy, winged demon-vampire named Marcus whose thirst for blood is rivaled only by his hunger for power (and, apparently, by his affinity for black nail polish). Marcus is in search of one of two keys that will unlock the coffin of his werewolf brother, William, with whom he wishes to breed a new race of hybrids (homoeroticism and incest notwithstanding). Michael and Selene have one key, while Marcus’ estranged father (a papal Derek Jacobi) has the other. Hell-bent to get at least one of them, Marcus chases the rest of the characters around obscure locales, mercilessly mangling and killing anyone in his path.

The fight scenes in “Evolution” range from gory to unbearably gory. But the climactic showdown between Marcus and Selene, William and Michael, is thrilling and well worth the cheap butchery leading up to it.

A few other moments do stand out in relief to the film’s dreariness, the first being some self-sacrificial wrist-to-tongue action between Selene and Michael, a setup reminiscent of Buffy and Angel’s classic “Drink me” scene. This tenderness foreshadows soft-core sex that, for anyone with hormones, is satisfyingly erotic, especially when the only other nudity in the film usually takes the form of blood-splattered breasts.

“Evolution” is not so much a sequel to as it is a continuation of “Underworld,” à la “Kill Bill Vol. 2.” It manages to wrench from its predecessor an intended existence, as if the first film were only created in order to breed a second, one that would itself be a sort of hybrid, a test-cross between invention and repetition. The result is not a complete failure, but a failure nonetheless.

By most conventional standards, “Underworld: Evolution” is a bad film. Writing, directing and acting are all inferior to forbears like “From Dusk Till Dawn,” “The Matrix” and even “Blade.” And as part of a genre that permissibly breaks the rules of storytelling to make room for rogue originality, “Underworld: Evolution” remains special only in the sense that it has an extra chromosome, one full of genes coded for bad taste.