Ten-year-old César is an only child. Césars father is a loud, irascible man; his mother, on the other hand, is quiet and loving. César is a fairly shy boy who has just one friend, Morgan. César is secretly in love with Sarah, the prettiest girl at school. Not that hes told anyone about his feelings. In fact, César isnt much of a talker at all. An introvert, he prefers to observe the world around him and is happy to think his own thoughts. One day, he sees a policeman with his Dad; shortly afterwards his parents have a row, which leads César to believe that his Dad will have to go to prison. The news of Césars fathers disappearance spreads like wildfire in the school playground and, all at once, César finds himself the centre of attention. But then, suddenly, his father comes home again. It turns out that he was away on business. His fathers reappearance has serious consequences for César; not only do his classmates now think of him as a notorious liar, but the headteacher is furious, too. Finally, his fathers reaction is to mete out the worst possible punishment imaginable: César will be sent to spend a week with his grandparents. For César, this is worse than banishment or exile. Césars relationship with his parents deteriorates still further when Morgan slips off to London one day, hoping to see the father he has never met. César and Sarah decide to head off for the Thames to help their friend. All at once, the children find themselves in a country where they dont speak the language, in a city theyve never been to before, in search of someone completely unknown to them. And this is where their real adventures begin.