Henry Allen's dog is missing - and he thinks it's been eaten by a dragon! On the night the dog disappeared, Mr. Allen swears he saw a huge dragon slither into the sea caves beneath his cliff-top house. Could Mr. Allen really have seen a dragon? The Three Investigators doubt it, but they're determined to find the missing dog. That means exploring those dark, dangerous caves>>> And whether or not Mr. Allen's dragon is real, something terrifying and deadly is lurking there!