Alyson Hannigan

On Feb. 22, former “Buffy the Vampire Slayer” star Alyson Hannigan leaves demon-infested Sunnydale behind for the mystery-filled seaside town of Neptune, when she makes a guest appearance on the UPN Tuesday-night cult hit “Veronica Mars.” She’s currently scheduled for one episode, with a possibility of the role becoming recurring.

Hannigan plays Trina Echolls, the struggling-actress daughter of town resident and movie star Aaron Echolls (Harry Hamlin), and stepdaughter of his wife, Lynn (Lisa Rinna, Hamlin’s real-life spouse). This makes Trina the half-sister of Logan Echolls (Jason Dohring), the high-school foe of teen sleuth Veronica (Kristen Bell).

In “Russkie Business,” Trina, who was a no-show at the memorial service for Lynn (who apparently committed suicide by jumping off a bridge) suddenly appears, raising new questions about what exactly happened to her stepmother.

“It’s a pretty dramatic episode,” Dohring says. “A lot of heavy stuff happens.”

On “Buffy,” Hannigan’s character, Willow Rosenberg, went from gawky computer nerd to powerful lesbian witch, and that versatility serves her well on “Veronica Mars.”

“Originally,” Dohring says, “they didn’t have her being snippy, but it’s cool for Alyson to have a bit of an edge. It’s a good match for me, because I’m snippy on the show.” The appearance of Trina is part of a larger story in which Logan questions his mother’s fate.

“He’s trying to find his mom,” Dohring says. “We don’t know what happened. In future episodes, he may enlist Veronica’s help.”