Of all the wannabe Harry Potter successors Hollywood studios have churned out over the last decade, this is one of the few that have actually managed to get beyond a first installment. The first movie about Percy Jackson, the teen who discovered he was a “half-blood,” the son of the Greek god Poseidon by a human woman, and sent off to a camp where along with others of his kind he could be protected and trained, was subtitled “The Lightning Thief.” Based on the initial installment in Rick Riordan’s series of popular young adult books, it pitted Percy and his friends—brainy Annabeth (Alexandra Daddario), a daughter of Athena, and the half-goat satyr Grover (Brandon T. Jackson)—against handsome Luke (Jake Abel), a son of Hermes, who’d stolen Zeus’ powerful thunderbolt with plans to use it to inaugurate a new era. It wasn’t just episodic but loaded down with tedious exposition, and it was stymied by director Chris Columbus’ leaden approach.

This second entry, “Sea of Monsters,” marks a few changes from the first. Anthony Head replaces Pierce Brosnan as Percy’s mentor, the centaur Chiron, and Stanley Tucci has been added as Mr. D—for Dionysius, the god of wine—as the head Camp Half-Blood (there’s a pretty good joke about how he’s being punished by Zeus for a previous indiscretion). Percy also gets a half-brother in the form of Tyson (Douglas Smith), a clumsy Cyclops who’s looking for a family. And while it’s similarly episodic, it has more action and moves along better, thanks to director Thor Freudenthal and editor Mark Goldblatt.

As to plot, the movie is basically a teen version of the Argonaut story, which begins when a group of nasty Cyclopes destroy the protective perimeter set up around Camp Half-Blood by a tree that Zeus created from the body of his daughter Thalia, who was killed trying to reach the place. After consulting the prophetess of Apollo who’s apparently kept in the attic, Percy decides that the only way to restore the Thalia tree to life—and bring the barrier back—is to send a mission to the titular sea (known to humans, we’re told, as the Bermuda Triangle) to find the Golden Fleece, which has life-restoring properties. Mr. D decides to act on this, but chooses a team led by Clarisse (Leven Rambin), a daughter of Ares who’s Jackson’s great rival for leadership among the students. But Percy and his chums, including Tyson, decide to take up the task as well, though without authorization.

The quest takes up the rest of the running-time. It turns out that Luke is once again behind all the mischief, seeking the Fleece in order to restore the Titan Cronos, the father of the gods, to life so that he can overthrow Zeus and allow a new generation to assume power. A series of adventures follow, involving a wild taxicab ride, a meeting with Hermes and some mild oceanic adventures—including a Jonah-like stint in the belly of a beast where some zombies also reside—before the crew winds up at an abandoned amusement park where a hungry Cyclops—and the tomb of Cronos—are to be found. There they have to face off against Luke and his army of the discontented.

This is hardly deep stuff, or a faithful rendition of the Greek myths: it’s more like a cross between Harry Potter and Ryan Gosling’s “Young Hercules” series. But it’s a reasonable modern equivalent of Ray Harryhausen’s cherished boys’ adventure movies, not just the mythological ones but the Sinbad series as well. Dads and granddads who remember those fondly will appreciate the nostalgia it brings. And the youngsters should enjoy it as well, since it features a passel of appealingly heroic teen types (a couple of them female for the girls), pretty good special effects, messages about the importance of family that aren’t too heavy-handedly transmitted, battle scenes that are fairly exciting without becoming overly nasty, and a few dollops of sentiment applied without much treacle. Lerman makes a pleasant if somewhat bland Jackson, and Daddario is an engaging partner for him. And while Jackson once again forces things in trying for laughs, Smith makes a nice addition to the crew. Among the others Abel is no more than a standard-issue young villain and Rambin shrill as Clarisse, but Tucci and Fillion seem to be having a field day in their brief appearances. Shelly Jackson’s cinematography is fine, with the3D providing some nifty moments, and Andrew Lockington’s score adds to the action without being at all memorable.

In all, “Percy Jackson: Sea of Monsters” improves on the first movie, making the obvious promise of another sequel at the end not completely unwelcome. As a boys’ adventure tale, it’s not awful, but not particularly wondrous either.