When Ian and Julie moved into their Surrey cottage nearly 20 years ago, they discovered a curious wall painting in one of the bedrooms. They had no idea who put it there. Julie wanted to get rid of it as it didn't match her decor plans, while their four-year-old wanted to colour it in with her crayons.Ian was about to paint over it when an elderly neighbour popped round and introduced himself. He'd heard from one of the previous owners of the cottage that the work was by famous abstract artist Ben Nicholson.Ben Nicholson was one of the greatest British artists of the 20th century. Born in 1894, he became a key figure in the European avant-garde. He was a pioneer of modernism who rejected conventional ways of painting and experimented with bold new means of expression. In a career spanning more than six decades, his abstract works were among the most influential in British art.Over the years, Ian and Julie have shown the work to experts from the art world, but they have doubts. Is this work by the master of British modernism, or is it an artful copy by a follower?Getting to the bottom of the mystery has recently become a matter of urgency. Ian is a builder, and he's midway through constructing an extension to the cottage. He's concerned the building work might put this potential masterpiece at risk of damage and destruction. If the team can prove the painting is genuine, it could be worth £200,000. But this poses a dilemma for the owners - if it is by Ben Nicholson, then a sizable chunk of their bedroom wall might have to be removed in order to preserve the painting.How could a work by Ben Nicholson have ended up on Ian and Julie's bedroom wall? To find out, Fiona and Philip need to look into the previous owners of the cottage. Did they have any connections to Nicholson, and if so, can it be proved that Nicholson visited the cottage? Fiona heads to the Tate to delve into a vast collection of Ben Nicholson's letters. Meanwhile, Philip enlists the help of the Courtauld Institute to try to find out how the wall painting was constructed and what it's made of.This thrilling investigation takes us to across the country on the trail of a master of British modernism. Is the wall painting in Ian and Julie's house a genuine work by Ben Nicholson, and if so, can it be safely removed from the cottage?