Seeking to recover the shards of King Arthur's sword Excalibur before Rex Major (the ancient sorcerer Merlin in a modern-day guise), Ariane and Wally have traveled around the world, but their quest is not over. Thanks to Wally, Ariane knows her mother is alive--and that Rex Major is trying to find her. In The Cave Beneath in the Sea, Ariane and Wally race to the Caribbean as they try to find Ariane's mother and the fourth shard of Excalibur before Major does. As they struggle to stop Major, Ariane and Wally must make the most difficult decisions of their lives.