A film - not only for school leavers - about one of the most responsible and professional jobs on British Railways. Practical work in shop and signal box, on gantry and by trackside, coupled with instruction in mechanics, electricity, electronics and draughtsmanship, lead the apprentice into the intricacies of design, the excitement of research and experiment, and the intense satisfaction of being in on a big 'changeover' from old semaphore signalling to a new colour-light scheme