Published in 1976, not quite fifty years ago, this quick-paced, kidnapped kids, mystery, thriller is a pretty good story for what it is: a bit of a bubblegum, afterschool, TV special. It's a kids book after all, written by Geoffrey Household. He was a British writer of some minor note, obviously enamored with John Buchan's writing style (which is not a bad thing IMHO).
A young boy named Mike is returning home one evening, when he notices a famous actor at a bus depot writing some information on a bulletin board. He asks the distracted actor for his signature, but the actor gives the kid a "brush-off". Unfulfilled curiosity causes Mike to look at what the actor wrote, a series of numbers that look like a telephone number. The child writes them down and gets snatched for his inadvertent nosiness.
When he awakens, he finds himself in a dark place with another child, Carrie, daughter of the actor that wrote his phone number down so that the kidnappers could contact him by phone about Carrie. The kidnappers begin their questioning of Mike with an initially warm demeanor, but are very disappointed to care for 2 children, when they really only meant to have one. This has changed things and any favorable outcome, may now no longer apply.
It's a good thing that Mike and Carrie are together. Although they never met each other before, the boy from a family of modest means, and the girl from a family of affluence come equipped with a set of complementary skills which help them cope with the unique situation. The story is not as profound as I would have liked, but overall, a good, fun, middle-of-the-road kid thriller.