This is one of Blyton's shorter novels, and while it was written much later in her life, there does not seem to be much difference from many of the other adventure stories of hers that I have read. In fact this book seems to be very much a shorter version of your average Famous Five adventure.
The story is about a brother and sister who in a previous story made friends with a couple of wealthy orphan twins (though this seems a little odd because orphan and wealth generally don't go together). Currently they are being looked after by their grandmother, however she died and they must now be looked after by a guardian. As they are wealthy it is very clear that an orphanage is out of the question. However, while they have inherited their family's wealth a part of that wealth ins an heirloom that is a cursed giant ruby that was brought back from India a long time ago.
Obviously thieves have decided they want the ruby so the kidnap the twins and hold them hostage and demand the ruby as a ransom (as thieves tend to do when they come across a get rich scheme, though in reality children are kidnapped for much more nefarious reasons). However the twins' friends are aware of the situation (they read the newspaper – obviously it is not the Daily Mail because it actually tells them something useful) and when they receive a note from them asking for help, they immediately set out on another adventure.
It is interesting that Blyton doesn't just work with one specific group of characters, like the Famous Five, but explores different groups of young detectives. However there was nothing really all that noticeable about these young detectives that made them stand out from any of the others (unlike the Five-Finder Outers). Instead they seem to be a little bland and have really no personality.
This story is pretty much the same as all of her other children detective stories (though Blyton only ever wrote children's stories). The kids stumble on a mystery and using their wits and cunning (as well as a lot of luck) they manage to outsmart the bad guys, alert the police to the situation, and in return solve the adventure and become heroes. She did write an awful lot of books, but usually when you have done so, the quality does begin to degrade and you end up repeating yourself ad infinitum. ( )