The Famous Five 13 - The Mystery of the Graffiti

You know what graffiti is: those drawings and paintings that certain people remember to make on walls, which have often just been whitewashed.
Uncle João hates graffiti, but Álvaro doesn't — and, like him, many other students at Vila Rica school like to make paintings and write phrases on walls.
The worst part is that something very strange is starting to happen to these walls...
And something even stranger happens to the graffiti artists. No one knows it, but the threat is very serious, as it is the entire country that is in danger.
Therefore, the Gang of Four has to take action.