If you want to play a game that combines science fiction, military action and horror (but doesn't take itself TOO seriously, without being annoyingly comedic) this could be what you are looking for. This book is the player's handbook, so both GM and players need to be conversant with its content.
The Introduction begins with an overview of the current situation. It is now 2450 and Earth has been uninhabitable since 2200. Former colonies near and far flourish, some have joined the Colonial Dominion. These have contributed to the formation of the United Colonial Marine Corps, the military arm of the Colonial Dominion. Extraterrestial lifeforms are known to exist but only one alien sentient race is officially recognised as an ally. A brief explanation of what a role-playing game actually is follows.
The rest of the book is made up of three sections. The first, Mission Briefing, elaborates on the background and setting of the game. Next is Boot Camp, which runs through the character creation process and basic rules, and finally Rules of Engagement goes into much more detail about how to play including loads of hints and suggestions about how to play well, using your abilites to best effect.
Mission Briefing opens with the underlying concepts of the setting, on the grounds that to role-play a Colonial Marine well you need to sound like you are familiar with the universe in which your character lives. Different colonies take pride in different things, and many cling to vestiges of Earth culture even though their homeworld is long gone (the reason why being left to the GM, by the way). Some colonies refuse to join the Colonial Dominion, but they are regarded with suspicion at best if not classed outright as rebel scum. The Colonial Dominion is not a centralised government, but a federation of independents. We also hear about communications, space travel and various kinds of human modifications... and 'dupes' (mechanical humaniod duplicates, or androids). There are mecha pilots, psychics and sensitives, too.
Next this section covers everyday tech, the stuff most people know how to use even if they don't know how it works. Holoboards and holoterminals replace computers and telephones, artificial intelligences and artificial gravity, all these and many, many more. Space travel is also covered here, along with stasis pods and other devices.
Moving on, Boot Camp opens with an overview of the rules, so that you can make informed choices when creating your character. It begins while explaining that characters are defined by their Abilities, and that the game itself is made up of Action Scenes and Narrative Scenes. Each scene is made up of Beats - three for a Narrative Scene and as many as the GM sees fit for an Action Scene. In each Beat, a character may do two ability-related things. It sounds a bit mechanical, but it actually flows a lot better than it sounds on paper once the group gets used to it. There are two ways of using Abilities, Active and Passive. Active uses are resolved by rolling 1d6 and adding the relevant Ability statistic, Passive ones by adding +3 during a Narrative Scene or +1 during an Action one, and may only be attempted using an Ability that you've actually put points into. Either way, your total is matched against a Target Number (which can be an opponent's total if it's an opposed action - fighting, or untying a knot someone else tied, whatever) and if you exceed it you gain Success Levels, the more of these the better you do at whatever you were attempting. Of course, there are a whole bunch of modifiers and conditions that may be applied, but that's the core of the system.
Then we get down to the business of creating a character. There are eight Abilities and each covers a broad array of thematically-linked actions. The actions include just about everything you'd expect a soldier to be able to do, although the Ability names themselves are a bit silly - 'Drop and Give Me Fifty' is the Ability covering physical endurance and athleticism, for example. There are also Aspects, which can have various modifying effects, and help you hone a character to the particular image you have in mind. To decide your Ability stats and any Aspects you decide to have, you get twenty points to spend. Abilities range from 0 to 4. There's a huge amount of further detail to explain just what everything is and how it works in game, as well as an armoury-full of gear. Some is part of your standard load-out, other stuff you might need to pay for. There is an array of sample characters to either pick up and play or to use as templates for creating your own. Each is 'named' for the defining nature of that character, you will need to come up with your own name and background if you want to use them. There's more: ranks, physical and mental trauma - and if you like reading tables of horrible outcomes you are in for a treat - and more.
The final section, Rules of Engagement, gets into a lot more detail about how the game actually works in play, starting with social situations and how you can use apposite Abilities to navigate your way through them. Plenty of examples here and throughout, so by the time you've finished you should be conversant with all the things your character can do and how to use them to best effect during play. Want to search a location, or defuse that bomb you just found? This is where you find out how. There are plenty of ideas for combat here too, and you can even pick up some tactics ready for the battlefield. The aim of the game (apart perhaps for survival, but do you really want to live for ever, Marine?) is to achieve glory in some manner. When you do that you earn Glory Points, which are used to develop your character's abilities later on. That doesn't necessarily mean being super-courageous, although that certainly counts (and might earn you a medal as well), it can also be for advancing the plotline, coming up with good ideas or even making the GM laugh!
This book provides an extremely comprehensive introduction to the game and how to play it, essential for anyone wanting to play I Love The Corps... go on, you know you want to have a go at being a Marine!
"Watch those corners..."