Unfortunately the film becomes laden with details and loose threads as it moves along, and fails to gain steam or provide answers. There are side stories that are intriguing when introduced, but never pursued. Then there are "discoveries" which are made much too easily, one wonders why nobody made them before.
Aneurin Barnard who plays Mariah, the older brother in the family, appeared to be the next heartthrob-in-the-making; handsome, talented, appealing. But it isn't to be as I doubt many people will rush to this film. Michael Sheen is splendid as he hams his way through this film, but he is unmatched by any other players, mostly due to the troubled screenplay with which they contend. Sam Neill is wasted in a role of broad villainous strokes that are tired and worn out.
Other characters appear and then aren't heard from again, supposedly until the sequel. By the time this film ends, there isn't a coherent TEAM of triumphant adventurers over a defeated foe, just kids who made it through a harrowing experience with the help of some grown-ups who are never strongly defined.
The film shifts abruptly from classic adventure to science fiction, which is such a bumpy transition, I was thrown off.