Single-player mods for Half-Life 2 are relatively few and far between. It'd be easy to pin that on amateur developers lacking the skill or will to create compelling plots and characterisations, but frankly the shockingly awful attempts at storytelling in so many professional videogames puts the lie to that. Simply, a single-player game is an awful lot of effort for not a lot of reward. To see a few dozen servers consistently doling out your multiplayer mod to a few hundred perfect strangers is infinitely more satisfying (and potentially lucrative) than some guy maybe popping on your forum to say, "Teh girl charactar was sexey lol."
Metastasis (the first, three-part chapter of a planned ongoing Minerva series) is one of the few that dares. It's been exceptionally well-received in some quarters, and a cynical mind might attribute this to there being something of a lack of choice. Not of single-player mods, but of single-player mods made with this degree of quality. At times, this approaches Valve standards of experience-shaping. If you desperately want more Half-Life 2, this will certainly shorten the ever-growing wait until Episode 2.
To a certain extent, this even outdoes Valve at its own game. Its approach to level design is one of total efficiency - your adventure for each of its two chapters is not made of small, linked level-boxes with a loading screen between each, but rather just one construct without an inch wasted. That building or watchtower you see at the start of the level isn't just background decoration - you will end up there later. As much as is possible is packed into both the horizontal and the vertical axes of each map, making for a seemingly sprawling single level that makes Valve's own approach of shoot for five minutes, wait to load a new section, shoot for five minutes and repeat almost laughable. There are two levels of Metastasis available currently, with a third due soon and a second chapter, Chronoclasm, due soon, so it'll be fascinating to see how far this approach can go.
Metastasis' other hook is its plot, which is admirably downplayed. The titular Minerva is a unreliable, sociopathic narrator of possible technological origin, eking out answers in barely comprehensible scraps and based unquestionably upon System Shock's SHODAN. Frankly though, the deliberately confusing dialogue isn't as clever as its writer probably thinks it is, and it's hard to conceive of a satisfying pay-off to all this Lost-esque riddling. As the faceless Minerva is the only even remotely friendly presence you ever encounter (and even then only on-screen as text cues), oftentimes this technique is vastly more atmospheric than having Alyx or Barney magically pop up and spew 90 seconds of exposition at you.
Unfortunately, the mod's somewhat hamstrung by a tragic defaulting to the miserable find the door or button-to-activate-a-door tasks that made Doom 3 and Quake IV as dreary as they so often were. While the economy of the level design might be ingenious, its actual challenges are not. We don't play first-person shooters because we enjoy looking for doors, and it's sad that a mod that seems to otherwise have some understanding of how to make a good FPS doesn't recognise that.
Moreover, Metastatis really highlights the retrograde elements of trigger-based enemy spawns and artificially locked-off areas that make Half-Life 2 a less satisfying game with each successive playthrough. There are no new foes, and though thoughtful design and especially use of new music creates a suitably menacing atmosphere, there's no escaping that you're in something built from bits of City 17. Still, given HL2's own limitations and the lack of an extensive budget, this was always going to be restricted to a certain toolbox. Given its polish it can be forgiven for sticking so closely to established FPS rules.
The slickest mod here by far, and the one that you'd be least likely to guess from was the fruit of Half-Lifean loins. Which totally scuppers my plan to refer to it as "Half-Life - in space!" It's closest to Battlefield 2 if anything, involving as it does the violence-assisted capturing of spawn points on the map and vehicles you're free to take on a deathride at any point. The classes are fewer for the time being - mostly variations upon the assault theme - though the hacker, who's able to secure a spawnpoint to a greater extend than any other class can manage, is an interesting divergence.
What really ensures that this isn't simply Battlefield 3142 is that the vehicles aren't tanks or helicopters - they're proper, actual spaceships, ones wot fly and shoot lasers. When you hop in one, the interface switches entirely to one resembling the post-Elite space sim familiar to players of something like freelancer or the X games. It's rather more basic than those, but it's a dramatically different discipline to the on-foot combat. The controls change, you'll need to shoot at where you think the enemy is going to end up rather than where he is, and there's some super-cool death rays. The ships are necessary to destroy space-based enemy resource points (and similarly gravity-unboound foes) and to access other, capturable areas of the map, so victory is easiest won by a melting pot of space and infantry scuffles.
It's clever stuff, both tense and utterly frenetic at once, and feeling somehow totally unlike Battlefield despite sharing so many of its mechanics. The only serious criticism is that the ships feel a little floaty-light at the moment - there isn't a great sense of being a hulking slab of flying metal rather than a human being with noclipping turned on.
Still, it's a fresh and exciting creation, and out of all of the mods here feels most like you're getting a totally new game as a free bonus for what you spent on Half-Life 2. Also, spaceships.