Guides from the Westminster, City of London, Greenwich, and Clerkenwell & Islington associations will be leading themed walks on the hour, every hour, between 10am and 4pm. Here's where to join them:
For the inside track on Nautical Knavery, meet at the Cutty Sark.
To discover Smithfield's Dark Side, be at Barbican station.
The City's Finest will be exposed, if you're waiting on the steps of the Royal Exchange.
Westminster's Secrets Revealed starts on Argyll Street, Oxford Circus exit 6.
No need to book, just turn up. The walks last 60 minutes, so if you're early and organised, you can get round all four.
Local London Guiding Day is on Saturday 12 October from 10am-4pm. All walks are free.