Percy Jackson’s friends urge the teenage demigod to change his destiny if he doesn’t like how it’s playing out. Thor Freudenthal, director of “Sea of Monsters,” apparently missed the memo.

“Monsters” is the follow-up to the Chris Columbus-directed “Percy Jackson and the Olympians: The Lightning Thief,” generally regarded as mediocre and derivative of Columbus’ earlier (and similarly mediocre) Harry Potter flicks. Twentieth Century Fox brought on Freudenthal (“Diary of a Wimpy Kid,” “Hotel for Dogs”) to do the second installment of Rick Riordan’s bestselling young-adult fantasy adventure series based on Greek and Roman mythology.

Freudenthal succeeds in focusing on the characters and not overdoing the CGI stuff. The problem is that there’s not much to the characters. Combined with a rushed plotline short on heart and laughs, “Sea of Monsters” is lost by the time Percy and his pals get to what we humans call the Bermuda Triangle.

Percy (Logan Lerman), son of Poseidon and a mortal mom, needs to retrieve the Golden Fleece to save the poisoned tree that protects Camp Half-Blood. First he must vanquish a sea of mythological creatures pulled from “The Odyssey” and “Jason and the Argonauts.” There’s also a gang of rebel Half-Bloods led by Luke (Jake Abel), son of Hermes, who want to use the fleece to resurrect Kronos, their Titan grandfather. Percy also is burdened by a prophecy that says his destiny is to save or destroy Olympus, home of the Gods. Oh, and there’s a subplot involving a surprise half-brother who’s part-Cyclops.

Viewers older than 10 and unacquainted with the first movie or the books may be annoyed by the lack of context. We’re not exactly sure why everybody’s out to kill the kiddies at Camp Half-Blood, or what the rebel Half-Bloods’ beef is, or how exactly Poseidon conceived a Cyclops.

The mythology is glossed over. When the gang arrives at the cave where the Cyclops Polyphemus guards the fleece, Clarisse LaRue (Leven Rambin) points out the boulder Odysseus used to trap Polyphemus in “The Odyssey.” The others stare back at her dumbly, which makes Clarisse defensive about having read the myth. It won’t exactly encourage kids to give Homer a chance.

Coming off “The Perks of Being a Wallflower,” Lerman brings some authentic brooding and gentleness to Percy. Kids may also learn about tolerance and sibling love from interactions between Percy and his Cyclops bro (Douglas Smith).

Otherwise, the acting is mostly unconvincing. Clarisse picks on Percy with relentlessly forced sarcasm and hostility, and Luke’s dependence on a vacant stare makes you wonder why they couldn’t have just created another computer-animated villain. Supporting actors Stanley Tucci and Nathan Fillion have a good punch line or two, but otherwise the book’s humor fails to translate with both kids and their parents.

It didn’t have to be this way. If Fox is fated to adapt all the Percy Jackson books to film, one hopes they’ll find a director who can save the series from a bad ending. The Greeks called those tragedies.