The Famous Five and the Z-Rays

I loved the Famous Five books when I was a kid. I don't recall much about them anymore though. What I do remember about this one was that it was a bit of a far-fetched sci-fi + amateur spy tale that involves a powerful new invention that must not fall into the wrong hands.

I'd recommend this series for kids who love adventure stories coupled with mystery and detective elements. It's like a younger version of the Hardy Boys or Nancy Drew.