Proton beam therapy is the one of the most technologically advanced though expensive cancer treatments in the world - but it has the potential to save the lives of children with otherwise incurable cancers.Over two years, Horizon follows the engineers, scientists and medics as they race to build two new centres, one at the Christie Hospital in Manchester and one at University College Hospital in London, as well as following as the first children awaiting the lifesaving treatment. This is one of the most complex challenges the NHS has ever attempted.