Opposing Force adds upon the already revolutionary game, and is arguably even better than its predecessor.
As the title implies, players will be introduced to Corporal Arian Shepard a marine from the Hazardous Environment Combat Unit that's sent into the Black Mesa Research Facility to deal with a mysterious biological outbreak and to eliminate rouge physicist Gordon Freeman. Opposing Force retells the story of Half-Life albeit from this change in perspective. And surprisingly the story blends in amazingly well with that of the original. As you arrive at the Facility, it becomes immediately clear that things have gone horribly wrong: unidentified flying objects intercept Shepard's osprey and destroys it forcing it to crash land. Shepard is then carried across the chaotic surface of the facility before being relocated to the infirmary section of the facility. The developers have done extremely well in retelling the story from another person's perspective but never make it feel as if you're in a different universe than that of the original and whilst Shepard is never aware of Freeman's location, Gearbox have done well at blending the story in with the original game, giving you hints of Freeman location and giving you the ability to revisit some of the more iconic locations in the facility from a slightly altered perspective. Your path even converges with that of Freeman's, which consistently shows you that you're still in the same universe and fighting the same epic struggle for survival.
Unlike Half-Life, whose story seemed to revolve around the prospect of ending the invasion, Opposing Force's story seems much more epic and more about survival than the need to end the invasion. Some gameplay and story aspects from Half-Life are even further expanded upon in Opposing Force including the Black Operatives and Gearbox has managed to include their own aspects. It soon becomes apparent that these Black Operatives are intending to end the crisis in a surgical strike and it's up to you to stop them from succeeding. The story's epic feeling compels players to keep playing.
Like Half-Life, Opposing Force is able to immerse the player by its consistent success at delivering surprises. Each level is different from the last and you never get the feeling as if you've done something before, or if something is done too much. Whilst Opposing Force is only one third the length of the original the game comes full with original content and gameplay. Unlike other games which tend to include hours of drearily similar firefights, everything in Opposing Force is different than the last. Overall you should be able to finish Opposing Force in little more than 10 hours or so.
Also like Half-Life, Opposing Force's gameplay is very puzzle orientated which means that on most occasions the player must solve a logic puzzle that will need to be solved before you are able to progress. These puzzles are always different and are located in just the right places so that it doesn't get repetitive. Unlike Half-Life where you could recruit up to a maximum 2 scientists or security guards, Opposing Force allows you to recruit up to 4-8 marines. These Marines are considerably more durable and effective in combat than scientists and fortunately they have the same advanced AI that made them such formidable opponents in Half-Life. Against the intelligence of the enemy along with the advanced AI of your marines some of the fire-fights that you get into seem amazingly fierce and realistic. However the job to keep your allies in line does tend to get a bit repetitive when they fall out of line or fail to accurately follow you. This can get rather frustrating at times when you need the additional help.
Opposing Force's arsenal is amazing to say the least, with an additional 9 weapons included on top of the original Half-Life, and just like the original each weapon is divided into its category ranging from melee weapons, to standard guns, to assault rifles, to biological weapons and experimental weapons and finally military issued fire-arms. Each gun has its own advantages and disadvantages, keeping the gameplay balanced but always making you choose between the right ones in a certain situation. One especially good addition is the Dispalcer Cannon. Whilst not only having the ability to vaporize enemies, the secondary function acts as a portable teleportation unit, giving you access to areas of Xen and Black Mesa that would have otherwise been inaccessible.
Opposing Force keeps all of the same enemies that were from Half-Life but also introduces an entirely different race known s Race X. This largely mysterious race has its own set of critters ranging from bipedal like dogs with scythe like arms to lumbering beats of amazing strength similar to the Gargantua. The game also introduces a new king of Super Zombie.
Opposing Force doesn't come with any particular Multiplayer component although the game does come with a set of unique death-match style maps that can be fought in over LAN connections and over the internet. Most of the maps have been outfitted with the new weaponry and concepts. However it's the single-player campaign that remains the best reason to get Opposing Force.
Whilst the graphics isn't considerably different to Half-Life there are some noticeable differences. One minor flaw with Half-Life was the lack of diversity when it came to character models: usually there were about 4 different scientist models, a single security guard model, and 3 types of marines. Opposing Force, whilst keeping the same models, adds even greater diversity. A new security guard model nicknamed Otis is included: a fat security guard that always seems to hang out near vending machines. 4 distinct Marine types are included such as a Medic who can heal your health, an Engineer who can help break down locked doors, a Demolitionist and a standard Marine. This minor improvement helps add to the immersion so that you don't always think the scientists and such have been cloned.
Despite being only one third the length of Half-Life, Opposing Force still manages to send players on a massive ride of surprises, story and action that will reel you in and capture you for hours of entertainment and excitement. In short, the best First-Person Shooter game in history has been made even better with the greatest expansion pack ever made.