Built by Collett in 1927 after pressure to restore the GWR's pre-eminence in motive power and cope with increasing traffic post-war to the Devon and Cornwall holiday resorts, the thirty Kings were the final development of the Churchward Stars and the 1923 Castles and remained on top-link main line duty until their final replacement by the 'Western' class 52 diesel hydraulics in 1962.

The book includes an insight into the thinking of some of Collett's senior staff at the end of the 1930s and the eventual transformation in the latter years with redraughting and double chimneys. As well as describing their design and construction, the book covers comprehensively their operation and performance backed up by many recorded logs on all main GW/WR routes over which they were permitted. The author had close experience of the class when working at Old Oak Common between 1957 and 1962 and includes a chapter of his experiences with them including many footplate trips (as a management trainee, he was greeted with glee by many firemen who would hand him the shovel). The book includes over 250 photographs of which 100 are in color.