EDITOR'S NOTE: This review contains minor spoilers for the original Half-Life 2. If you are yet to play this utterly compelling king of shooters then read no further - instead, click on the link below for our Half-Life 2 review, then buy the game as soon as you can. You won't regret it!

After spending hours bitterly fighting, after tirelessly setting in motion the events for a full scale revolution and having spent every last ounce of energy toppling a totalitarian regime bent on stunting the growth and prosperity of the entire human species, Half Life 2's ending left all those who'd experienced the journey that proceeded it with a sense of dismay, and a feeling that can be summed up perfectly in three words: "Is that it?"

As endings go, they don't come as anti-climactic as Half-Life. Having already left the orignal game with an ambiguous conclusion, one that saw protagonist Gordon Freeman accepting a job offer from the omnipresent G-Man to work as some sort of Time Commando to be put on ice until the next alien invasion (or dead if you happened to choose the other ending, but that choice negates both the sequel and this episode altogether) Valve went one further with Half-Life 2 by ending it on a bit of a cliff-hanger.

Who exactly is the G-Man? Where did the Combine come from? And my God, tell us, did Gordon and Alyx survive that explosion? Maybe it was a means to provide answers to the many questions left unanswered after Half-Life 2 that Valve decided to abandon plans to develop the straightforward (and more sensibly named) expansion pack Episode One was going to be in favour of trying their hands at episodic gaming. The intention here is that Valve hope to tie up any loose ends left by the previous games in a trilogy of episodes, therefore preventing us waiting another six years for the next sequel.

It's difficult to write a review for Episode One without avoiding certain spoilers, but this introductory episode at least begins by answering the question that left us all hanging after Half-Life 2. Gordon and Alyx didsurvive the cataclysmic explosion of the Citadel - how or why is never explained and will likely be elaborated upon in later episodes, but the reasons for their miraculous escape matters little, as the pair soon find themselves out of one disastrous situation and straight into another, as the Citadel kicks in one last time and preps itself to self-destruct, the concussive effect of which will destroy the entirety of City 17 and all those left inside. It's within the crumbling remains of this metropolis that Gordon must make good his escape, discovering certain secrets that point to the true intentions of the Combine along the way. However, Gordon is not alone in his journey, as Alyx now teams up with the gun-toting, spectacle-wearing scientist providing him with some much-needed company.

It's this new AI co-operation that's the main addition to the game; providing Gordon with a companion adds a nice new dynamic to what has largely been a solitary experience thus far. Valve has avoided the usual pitfalls associated with AI teammates by actually making Alyx useful. While other games struggle to make AI teammates anything more than a hindrance to your progression, Alyx is so adept at looking after herself that it can often feel like you're playing a co-op game. Although it is a little jarring at first to find yourself fighting alongside an AI teammate who isn't a complete meathead, it's the one thing that sets Episode One apart from its two predecessors. For once you're not going it alone, killing Combine and solving puzzles with muted responses; Alyx is there almost all of the time, providing verbal clues as to what you need to do next and where you need to go. She constantly alerts you to the presence of enemies and is skilled enough to hold her own in a gunfight, thus preventing you from feeling as though you have baby-sit her through every dangerous situation you encounter.

However, co-operation works both ways, and Alyx also needs help from time to time. This especially becomes apparent during the creepy opening to the second chapter, which is set in a maze of underground tunnels and sewers populated by headcrab zombies, antlions and other nasties. Still with no weapons of your own at this point, it becomes your task to guide Alyx through the darkness with nothing but your torch, lighting the way ahead and shining the light on creatures lurking in the shadows. Accidentally (or purposefully) switching your torch off leads Alyx into fits of hysterics, as her fear of the pitch-blackness and the monsters waiting to pounce settle in. With such a verbal partner it's difficult not to care about Alyx's well being, especially as she proves to be so darned useful throughout your journey.

While not perfect, and still succumbing to those occasional scripted sequences where Alyx somehow manages to scale objects that remain un-scalable by yourself, the new AI team system works remarkably well, and it'll be interesting to see where Valve takes it for future episodes. Perhaps teaming up with Dog, or maybe one of those Vortigaunts can take a break from not actually doing anything useful and lend one of its three hands; I'd even settle for partnering up with Lamar just for the hilarity that would likely ensue!

As this isn't an expansion pack, Episode One doesn't contain a great deal of new features, barring of course the additional AI companionship of Alyx. One new feature however is the inclusion of the High Dynamic Range lighting first showcased in the free downloadable level, The Lost Coast, the effects of which make it appear as though your eyes adjust to light as you enter and exit shaded areas, as well making the whole game look prettier. While it's no secret that the Source engine Valve spent years creating for Half Life 2 is starting to look a bit past its sell-by date, the addition of HDR give the game the visual boost its needs for it catch up with today's tech hungry gamers.

Valve use every opportunity to exploit this effect, and do so with a great deal of success. The opening chapter that once again sees you plunging into the depths of a now crumbling Citadel throw up some incredibly impressive effects, while the central core that powers the mammoth structure really is something that needs to be seen; providing your computer is powerful enough to run HDR that is. The game isn't short of other graphical improvements either; the constant stream of ash that billows down from the burning Citadel adds a nice sense of the utter decay of City 17, while subtly but noticeable tweaks to what were already vastly superior character animations help add a sense of realism to some of the characters, though it would have been nice had this effect been given to all characters instead of just the main cast.

One of the pitfalls of episodic gaming is naturally going to be length, and while Episode One manages to keep you entertained throughout its running time, it does only clock in at around five hours and can easily be completed within one sitting. Valve have paced things accordingly though, and playing to the strengths of both Half-Life and its sequel, they've taken the best of what they've learnt from those two games and crammed them into this episode. Everything is more tightly packed this time around; the action comes thick and fast, while those long scenic levels of Half Life 2 that saw you negotiating City 17's terrain in a series of vehicles have been scrapped altogether (though still may make an appearance in later episodes). Valve has successfully brought the chaotic feel of City 17 to life, as its infrastructure collapses. The AI this time around gets a chance to fight amongst itself, as the Combine, headcrab zombies and antlions all converge upon one another, leading to several very impressive large scale encounters with these different factions.

Spread across five chapters, the action barely lets up from the beginning of the second chapter, with a genuine sense of urgency driving you to escape a city on the brink of destruction. It isn't all action though, and responding to one of the main criticisms of Half-Life 2, Valve has added a hefty amount of new puzzles to change the pace. Some take the form of the familiar physics puzzles used throughout Half-Life 2, while others provide a cerebral challenge that requires more than simply shifting boxes around.

While they could have churned out a substandard expansion pack and coasted on the success of their already well established and very popular franchise, Valve has used Episode One as an opportunity to address problems with its previous games, the result being an experience that often feels better than the games that it expands. It's also nice to see the inclusion of a developers' commentary. Appearing as speech bubbles throughout the game, the commentary gives a great deal of insight into the process of making this episode, citing the ideas that Valve tried, as well as the designs they were going to use before testing led them to make changes. It covers everything from how scripted events are used to divert your attention elsewhere to the use of music to set a certain mood for some sections of the game. It's a nice, interesting addition that gives more than enough reason to play through Episode One at least once more.

As great as it is to get the chance to walk in Gordon's boots once again, finding answers to those questions and getting to see City 17 in a whole new light, Episode One can't avoid the nagging sense of disappointment. There's nothing remarkably new here, no new weapons to toy with, no new environments to explore and ultimately it's an experience that, while improved upon, isn't vastly different from what you've played before. This isn't much of a bad thing though, as Half-Life 2 is an excellent game and in some ways Episode One exceeds it, but there's always that feeling that something new would have enriched the experience even more.

Regardless of what hasn't been included, Half-Life 2: Episode One is a superb addition to the Half-Life saga that greatly improves upon the old ideas already pioneered by Valve. While it can occasionally feel as though this episode is merely testing to see how well you've remembered how to fight the Combine, for the most part it's a non stop thrill ride through some best bits of the both Half-Life games, with the added inclusion of some incredibly smart AI companionship. Although Episode One may end with more questions than we were left with at the end of Half-Life 2, at least now we know that these will be answered - and soon. Episode Two is already far into development and it's set to provide the Half-Life universe with new features. Add to this the shock announcement of two new multiplayer games (one of which is the long thought dead Team Fortress 2) that should address lifespan issues and Episode Two really can't come soon enough.

Reviewed by Kieron Giacopazzi for AceGamez (All Rights Reserved).