Mike McKenzie is a bored,self-made millionaire who is cheered to learn that art auctioneer Laura Stanton,the old flame he allowed to escape him,is back in Edinburgh. Mike is disdainful of private art collectors and is interested when art expert Professor Gissing suggests that they,along with banker Allan Cruikshank,devise a plan to swap priceless paintings from the national gallery and owned by a banking conglomerate for clever forgeries. The forgeries will be left behind,giving the impression that the thieves abandoned their haul when they fled. For a single day the gallery is open to the public under the Doors Open scheme and the trio plan to use this to their advantage. Mike brings on board another old friend,small time crook Charlie Calloway,but Charlie is in debt to a gang boss known as the Geordie and wants a painting to buy him off. The gang use a fake fire drill to pull off the theft and,despite getting locked in the gallery,are successful. Laura returns to Mike,who confesses what he has done but Gissing has double-crossed the others,absconding with all the paintings and leaving Allan,Charlie and Mike at the mercy of the Geordie. It is down to Laura to save the situation.