This book provides all that you need to run adventures using UNIT, the Unified Intelligence Taskforce (originally United Nations Intelligence Taskforce, back in the days of the Third Doctor when we first encountered them). UNIT is Earth's front line of defence against alien incursion, and can be a potent source of adventures with or without the Doctor being involved, especially for groups who enjoy a more military, combative approach than the Doctor's likely to tolerate! UNIT also hires researchers and scientists along with investigators, so there's plenty for characters who do not wish to be soldiers to do as well. If you are playing a game with the Doctor and his companions (or another group of time-travellers with or without Time Lord) UNIT may be encountered as allies or even adversaries during your travels, or like the Third Doctor, your group may work with them on a more regular basis. Or a single UNIT member may join the companions... Whatever you choose to do, this sourcebook will provide the basis for doing it.
All this explained, Chapter 2: UNIT History deals with the history not just of UNIT itself but of precursor attempts to defend Earth (or at least the UK) against alien invasions. UNIT itself was formed when the British Army struggled to repulse Daleks, War Machines and Robotic Yeti. Although the Second Doctor met them occasionally, a long-term relationship was formed with the Third Doctor, and more recent regenerations have interacted with UNIT upon occasion. This history is quite detailed and illustrated with some magnificent (but alas uncaptioned) stills from the TV show, and gives an excellent account of UNIT from its inception to the present day.
Next, Chapter 3: Modern UNIT describes how the organisation functions in the 21st century. This covers its organisation and goes into considerable detail on what you'd find in a typical UNIT base - if you are running a game based on UNIT it's likely that you'll need one to serve as a home for the group. Mechanically, a base - just like a character - has a series of Traits, good and bad, that define its general nature. As an example the UNIT HQ in the UK is described in detail. Next is a discussion of security clearances, for people and for documents, so you can give everything the appropriate feel... there are also some brief uniform notes for those characters who want to know what they'll be wearing. A few gadgets round this chapter out.
Chapter 4: UNIT Personnel provides details necessary to create a UNIT character from scratch. This includes ideas on the various sorts of teams that you might set up in a UNIT-focussed game, depending on the nature of adventures you want to have. Some archetypes are provided for those occasions when you need a UNIT operator in a hurry, or to use as a basic outline on which to construct your own character. There are also biographic notes and full character sheets for several notable UNIT characters who have turned up in the TV show.
Then Chapter 5: UNIT Tactics provides some expanded combat rules. The core rules regard combat very much as a last resort (much as does the Doctor), but UNIT gives an opportunity to have a few brawls... so here's how. So there's a fair bit about firearms and a few other things, and a piece on how to handle mass battles including how to involve individual characters, laying siege to bases and more.
Next is Chapter 6: Covering Up. UNIT would be much happier if the rest of us didn't even know about aliens, let alone what they are trying to get up to (like invading Earth), so here we learn how they keep a wrap on things... or at least try to. An Exposure system is included to cope with the times when the cover-up fails, with the assumption that knowledge of aliens is going to cause mass panic.
This is followed by Chapter 7: Gamemastering UNIT which covers everything from the sort of themes you might employ - loyalty, morality, fear - to the likely nature of a UNIT-based campaign. That can be defending the Earth or it might take a more exotic twist - perhaps the UNIT team has acces to a TARDIS or other time-travel device and goes out exploring. They could even provide protective details to important individuals, or guard alien captives. There is a wealth of ideas here, well worth reading if you intend UNIT to turn up at all in your game never mind be the centre of it. Then the chapter gets down to the nitty-gritty of designing and running UNIT-based adventures, addressing issues like having a military hierarchy which never sits easily in the context of a role-playing game.
Finally, Chapter 8: UNIT Adventures provides masses of material to get you going, with two complete adventures and several adventure outlines/seeds which you can develop further. Armed with this book, you'll be ready to bring UNIT to life in your Doctor Who game, whether you want them as a central part with the party as UNIT operatives, or just have them turn up occasionally in a convention Doctor and companions campaign. Pretty much essential reading...