The story was boring, and being a school text makes more so. It was written by someone who was a teacher and a promoter of English, and not by an established writer.
But in any case, I find it very interesting to look back at the story in an age where computers were a novelty preserved to large governments or research facility.
The plot is simple, a professor (Mastermind) is killed, and a secret agent (John Carstairs) is sent overseas to a remote island in the Agean sea to investigate. All computers around the world are stopping. A computer called D.O.T. (Data Overseas Transmission) is programming other computers. D.O.T. saves the secret agent and wants him to join him. The secret agent then types in a secret number found with the professor and kills D.O.T.
What is interesting is that the simplistic plot includes a lot of themes from other science fiction, such as "2001 A Space Odyssey", "The Matrix" and "I, Robot". This includes smart computers who rebel on their master humans and try to dominate the world.
Looking back over the decades and seeing something like that is surreal. We did not even know how a computer works in those days ... Perhaps made for a better story ...
See also: this review.