Charlie Horloge doesn't see his father very often, but when he's around, his dad makes him read history books and tells him to guard his possessions if he's in a strange place. Now he's fifteen Charlie wants to know more, and why the teachers always have trouble remembering him. Then the police come calling, and his dad vanishes. Later, spotting one of the strange policemen again, Charlie follows the man by jumping into a hole in the ground. He appears in an old- style city, where his smartphone turns into a notebook, and is promptly pickpocketed by a young girl. As if that wasn't enough, Ben Franklin is a journeyman printer nearby.

In UNTIMED, Charlie is told 'Boys are from the future, girls are from the past' by another time traveller. Clockwork agents from the future - like that policeman - are out to kill them. London, the grimy stinking city with foul water and fouler poor, could easily kill Charlie too. And a girl his own age is a teen mother, with a sharp 'Where were you last year?' when he makes a comment. If he can only go downtime - past - how can he get home? And how soon can he start? Will that change time?

I found the concept was a cross between The Time Traveller's Wife and Jumper - Griffin's Story but as both are for older readers, the tale may be new to young adults. I enjoyed reading about London when Fleet Street was a ditch and Oliver Twist could have been a real person, filching purses and sipping gin. The dirt is overwhelming so this tale is not for the squeamish, though instructional, and there is plenty of action. Other locations follow and nowhere seems very safe for very long.

Andy Gavin has put together a gripping tale of time travel, but scenes of teen sex, violence, drunkenness and death merit a parental advisory, given that young people like reading about slightly older age groups. At the same time the protagonist keeps insisting that life is different in his present, doesn't enjoy the drinking or violence and warns against drugs. The tale also gets more complex with history changing and back and forth jumps, so concentration is required. The prospect of the leaders of the American Revolution being hanged for rebellion makes this a controversial read but if you enjoy a good argument read UNTIMED and see how it might have occurred.