David Willis, of Dallas, Texas, wandered into Waterstones’ Trafalgar store and went upstairs to look around for around 15 minutes.
When he came downstairs, he found all the lights had been turned off and the store was locked. The store had closed at 9pm.
Just in case nobody believed him – he took a picture and posted it to Twitter pleading for help.
At 10.11pm he said: ‘This is me locked inside a Waterstones bookstore in London.
"I was upstairs for 15 minutes and came down to all the lights out and door locked. Been here over an hour now. Supposedly someone is on their way. #nofilter #london."
At 10.55pm things weren’t looking any better for Mr Willis.
‘Please let me out,’ he asked the Waterstone’s team.
His messages were retweeted more than 5,000 times by fellow Twitter users keen to help.
Journalist Gaby Hinsliff was concerned.