Seacastle

Nick and his little brother Thomas are just two ordinary Australian boys - well, except that Thomas is a bit more annoying than most younger brothers!  But when Thomas almost drowns in the lake, and Nick swims out to rescue him, something amazing happens.

They find themselves in a weird underwater world full of mermaids and other strange creatures - ruled over by the Queen of Seacastle, and her talking crystal, the Beacon.

The Seacastle and the whole mermaid Habitat are in danger of being destroyed, and it's up to Nick, Thomas and their new friend Catya to save everyone by carrying the Beacon back to their world... but their journey home is far more dangerous than they imagined, and they are in real danger of never finding their way home at all!