It runs about as long as a film, two and a half hours in Allentown, except you are in it and someone might betray you. In plain terms: your team explores on foot and takes on spy missions together. If you are the one organising: the missions give everyone something to contribute, there is no lecture to sit through, and you look very good for finding it. You will decode messages, photograph evidence, and keep a straight face in public. No experience needed. There may or may not be a double agent on your team. Step-free mapped paths all the way: 2.4 km, 4 stops and about 2.5 hours, at whatever pace your team likes. No guide, nothing to download. The verdict: better than a film, mostly because nobody shushes you, and you leave with the photos. Who it's for: friends, couples and families with teens; and private groups, from bachelor and hen nights to work teams and active over-55s. Book a private game for 1 to 8 players on a date you choose; a team of one works too. Bigger groups play as several private teams.