Get down to ground level as quickly as possible - no health, no tools, no enemies, no real story. Just you against gravity and geometry.

Start to finish, this is less than 5 minutes long, though my first completion took over 20 minutes of restarts (I avoided quicksaving/loading). I have since found myself loading up for a few minutes at a time to try and improve my speed, so it's definitely moreish.
The only let-down is Thief's sometimes janky environmental interactions - you may find yourself missing ledges you are sure you should be mantling, or bouncing awkwardly off walls as you hop down. But if you can push past those quirks this is a really fun gimmick mission with some replayability.