The couple were none other than the Queen and Prince Philip on a private trip to see a play in London's West End.
Only a handful of audience members at the production of War Horse saw the monarch – wearing a dark green dress – and her husband taking their seats in the front of the circle.
Four places had been left empty for the royal visitors and their security personnel.
It wasn't until the interval, when the lights went back up, that people began fully realise who they were. "There was some applause, which she acknowledged with a wave," said an onlooker.
The Queen seemed to enjoy the performance – which uses life-sized puppets to recreate author Michael Morpurgo's book about a young boy's travels in WWI.
She joined in a standing ovation with the rest of the audience as the curtain came down
It is thought the Queen and the Duke of Edinburgh regularly go on incognito outings. Their last trip was to see Billy Elliot in 2006.
A Palace source: "The Queen and the Duke do go out privately but we wouldn't always know about it. It allows them to live a bit more of a normal life."