Strange Adventure

Sam and Luke vanish: there one second, gone the next. That's the last thing Kitty Strange expected to happen while in her lounge after school with her best friend Lizzie, Luke's twin sister, and Toby, Kitty's twin brother. Where have her younger sister and Luke disappeared to?
Minutes later, Kitty, Toby, and Lizzie materialize in a parallel world, with just one clue as to where Sam and Luke are. What do they do next?
Launch a rescue mission, of course, that leads them to an academy for gifted children owned by Dr Ravenel Raven, an Aquaterran computing genius. Using a 'jumping machine' he stole from mysterious aliens, Dr Raven plans to visit parallel universes in search of a cure for his sister's incurable illness. But first he needs to get the machine working properly - at any cost.
Kitty, Toby, and Lizzie learn that in the first experiments humans have indeed been 'jumped' to Aquaterra but are having memory problems. And what's worse, in an attempt to repair their memories, they'll soon be jumped to Earth and back, which in fact may well scramble their memories beyond repair.
The countdown to save Sam and Luke begins - but in a hi-tech alien world where real and virtual are sometimes impossible to distinguish, with armed sentinels chasing them, and with the jumping machine to find, will they be in time?
A fast-paced and thrilling sci-fi adventure for middle-grade readers.