The mythology-rooted “Percy Jackson & the Olympians” franchise is fairly engaging stuff, unabashed “Harry Potter” knockoff or no. Author Rick Riordan’s popular kid-lit series made its screen debut three years ago with an adaptation of “The Lightning Thief” — directed by “Potter” alum Chris Columbus, naturally — and proved to be a fun, brisk, even intermittently educational fantasy spectacle. As the lead, Logan Lerman (“The Perks of Being a Wallflower”) skated through the action with easy charisma. If the movie had had a convincing emotional core, the imitation discussion might have been footnote material.
The new sequel, “Percy Jackson: Sea of Monsters,” delivers more of the same, on all counts. Dispensing with any rote civilian-world angst, the movie dives right into the latest challenge facing hoodied demigod Percy: An assault on Hogwarts — er, Camp Half-Blood — has fatally damaged a mystical tree that projects a protective barrier around the compound. Percy and his demigoddess gal pal, Annabeth (Alexandra Daddario), realize that the healing power of the legendary Golden Fleece is the answer.
But oh, all the gods-darned obstacles complicating their Fleece quest. Percy already questions whether his successful first-installment odyssey was just a fluke, never mind that he’s Poseidon’s son. Mentor-centaur Chiron (cleverly cast Anthony Stewart Head of “Buffy the Vampire Slayer,” taking over for Pierce Brosnan) lays a cryptic prophecy on him about the mission. Meanwhile, another camp counselor, “Mr. D” (Stanley Tucci, amusingly conveying Dionysian in leopard prints), hands the quest to Clarisse (Leven Rambin of “The Hunger Games,” playing irksomely confrontational, even for Ares’s daughter). And as it turns out, maybe scheming “Lightning Thief” nemesis Luke (Jake Abel) isn’t quite as out of the picture as we’d assumed. (In a related, throwaway cameo, Nathan Fillion trades on his Comic-Con cred as Luke’s dad, Hermes.)
Kinda, sorta undaunted, Percy, Annabeth, and satyr buddy Grover (Brandon T. Jackson) set out to retrieve their prize from the Sea of Monsters, better known to us mortals as the Bermuda Triangle. Percy’s newly revealed Cyclops half-brother (Douglas Smith, “Big Love”) tags along — more of that undercooked “human” drama finally resurfacing, as ineffectively as last time. The character does offer a lesson, though, in how to help manage a mid-level effects budget: Stick your Cyclops behind sunglasses, or spritz him with an appearance-normalizing potion.
Following a couple of underwhelming effects sequences in the early going, director Thor Freudenthal (“Diary of a Wimpy Kid”) finds his groove with a succession of flashy 3-D renderings. Highlights include a gaping-mawed, vertigo-inducing Charybdis, and a molten, climactically resurrected Kronos, the gods’ vengeance-thirsting father. They’re digitized riffs on the Sarlacc pit from “Star Wars” and the finale of “Raiders of the Lost Ark” — but as with the “Potter” cribbing, when it’s done well, it encourages “Percy” audiences to forgive the derivative chunks and thin emotion.