The entire purpose of "Slayers - Premium" is to reunite the bickering, food-mad, highly-destructive little fantasy team, and send them on another bizarre quest du jour. There's not much more substance to this frothy little tale of octopus rebellion and mixed-up language (and Xellos is there just to... be there), but overall it's a fun little diversion with plenty of "Slayers" charm.
Lina and Gourry are dining at a village famed for its octopus... only to learn that the octopus is tainted by evil demonic power, which makes Gourry spout gibberish Octopese. And when Amelia and a very reluctant Zelgadis arrive, they are quickly infected by the disease -- and though there's a spell that can cure them, the local sorcerer can't say the words!
So Lina and her incoherent pals venture down into the depths, and discover a hyperintelligent octopus who plans to resurrect their sea god, and take horrible revenge on all of humankind. And unfortunately, Lina has also become infected with the Octopese-disease... just in time for her Dragonslave to putter out. Will the octopus god destroy the afflicted town? And just what is Gourry saying when he says, "I love you!"?
"Slayers Premium" is basically like watching a slightly long, rather fluffy episode of the anime series (despite a blink-and-you'll-miss-it cameo by Naga during the climax). And it's basically packed with all the required gags that "Slayers" has always had -- Lina's savage beatings and flat chest, squabbles over food, Amelia's dramatic speeches, and the occasional Dragon Slave.
Beyond that, the plot is a pretty fluffy affair -- the whole idea is that octopi are enraged by humans hunting them, and their foppish hyperintelligent leader is planning to unleash a terrible threat on land-based life. So it has plenty of sword fights, magical attacks, Gourry fighting with "bullet time," and plenty of humor derived from the Octopese language (including all Lina's friends moaning, "FLAT CHESTED!" in unison, to express their dismay).
And the handling of the characters is mixed -- Lina is wonderfully bombastic and short-fused, while the hapless Gourry babbles when he isn't swinging the Sword of Light, and the much-abused Zelgadis is dragged on a culinary adventure he wants no part in ("Octopus is NASTY!"). Unfortunately, Amelia is portrayed as rather too oblivious and hypersensitive (she bursts into tears because Zel tells her to walk ahead?), and Xellos is basically there to tell Zel what's going on. Really, that's all.
It's worth noting that the voice actors in this movie are not the same as in the TV series, save for the excellent Crispin Freeman. The other range from good (Lina and Gourry's voice actors) to bad (Xellos sounds like a ten-year-old boy, and the "That is a secret!" lacks mockery).
"Slayers Premium" is not premium by any stretch of the imagination, but it's a cutesy little adventure story that entertains for a half hour. Amusing, if rather stretched out of shape.