There are few more iconic paintings in British art than "The Hay Wain" by John Constable, the 19th century landscape master. A picture with a direct link to this milestone in British art would be the Holy Grail for any collector, and the team is on the trail of a striking sketch that appears to depict the very same scene: Willy Lot's cottage on the banks of the river Stour. Now owned by a Gloucestershire businessman, the painting appears to have all the hallmarks of Constable's sketches - his more impressionistic preparatory works. If genuine, its value could be counted in millions. The trouble is, the picture has had something of a chequered past, with its authenticity questioned by the last generation of Constable experts. Can the latest advances in scientific analysis and deep research into the picture's provenance turn up enough evidence to prove that it is a genuine work by John Constable, taking a first step on the path towards discovery of one of the world's most famous paintings?