Set in the Old West, this adventure is fast-paced and ought to play out in one or at most two sessions. The introduction lays out all the background detail of what actually is going on (just in case the cover picture hasn't given it all away!), enabling the GM to run this as a sandbox investigation: here's a town, here are some problems, go find out what's happening and fix it! The adventure seems ideal for any Doctor and companions - or other exploring time travellers - party (apart from the photos there's no reason to set it in the Eleventh Doctor era!), but would work less well with games involving UNIT or Torchwood.
The adventure opens with the TARDIS arriving in the small township of Prosperity in the early evening, and provides plenty of descriptive material should the party wish to wander around exploring and meeting people. A good range of options are provided, with the relevant outcomes, you might need to suggest one or two to your players if they are not sure of what to do. They do provide plenty of avenues to discover firstly that there's something odd going on and then to find out what that might be and do something about it, and the nice thing is that the order in which things are done does not matter.
Another neat touch is that there are various groups all with their own plans and purposes, who are in the main getting on with them regardless - this is not a static scene awaiting the arrival of player-characters, but a dynamic situation with which they can intereact.
This should provide your group with an exciting adventure, and captures the essence of a Doctor Who adventure well!