I haven't read that book in a long time, but I think the gist of it was that Alex Rider lives in London with His uncle, Ian Rider, and his American babysitter, I forgot her name ...I think Jack or something that sounded like a mans name. Anyway, the uncle dies, supposedly due to a car crash. Alex investigates the suspicioous bank that he works for and after scheduling an appointment and meeting with one of the uncles associates he goes to great lengths to investigat the bank and finds out that it is actually not a bank, but is MI6. Basically they recruit him as a child spy to go after the guy that killed his uncle, Herod Sayle.... there is some other stuff, but if you want a better overveiw you should check out wikidedia, there is also a movie version of it I think.
