The cleaners of a huge John Constable oil painting of Waterloo Bridge try to solve its 200-year-old mystery, as they spend months slowly cleaning away the grime and the varnish. In Northumberland, the fight is on against an invasion of pests in the important taxidermy collection of Wallington's amazing Cabinet of Curiosity. At Britain's greatest surviving Victorian textile mill, two complex, fragile machines are taken apart for a deep clean.

Part one of two. The work of conservation cleaners looking after national treasures, beginning with those who look after railway carriages of the Golden Age of Steam. Plus, in Marylebone, the fragile Portland stone carvings of a church once frequented by Charles Dickens get what may be their first ever deep clean.