The Perkinski's are all having fun at the travelling fair with its freak shows, wild animals and Gran's gypsy potions. But things turn nasty when Jem witnesses a kidnapping and finds himself framed for a crime he didn't commit.

The boys are soon up to their necks in trouble down on the river Thames and it looks like it's up to little Billy get his brother out of deep water. But are the Perkinski's a match for the darkest criminal mind in London? A creature so evil and villainous he is known as The Snake . . .