Created by one man, Richard Seabrook, Prospekt is a kind of Half-Life 2 fan fiction. It's not a mod as such—you don't need to own Half-Life 2 or its episodes to play it—but it's set in Valve's world, and uses its enemies, weapons and assets. Presented as a follow-up to Half-Life: Opposing Force, you play as Adrian Shephard—recently rescued from the G-Man's stasis by the Vortigaunts, and dumped into Nova Prospekt to help Freeman. 

Half-Life 2 is one of the best shooters ever made, making it a strong launching point for another game. But, from Prospekt's first combat encounter, it felt to me like something was off. The breaking point came during a battle at the start of chapter five, when, in a fit of frustration, I quit out, loaded up Half-Life 2, and used the command console to check out a hunch. Sure enough, the Combine Overwatch Soldier and Elite—the two enemy types you fight throughout the majority of Prospekt—have been given significantly more health.

Unfortunately, more health doesn't just mean a greater challenge. It's bad enough that the enemies are so bullet-spongy, but it also upsets the flow of combat. Where, in Half-Life 2, you could reliably pump both barrels into a soldier and instantly move on, here that same act might not do the job. Prospekt was constantly nudging me off a rhythm I'd spent 11 years perfecting, but not in a way that felt interesting or subversive.

And there's so much combat. In Prospekt, a change of pace means walking a long corridor or pressing a couple of buttons. Some early lightweight puzzling had me hopeful for some variety through the campaign. Instead, the vast majority of this game is about shooting the same few enemies over and over again. Even the toolset is reduced. It's understandable that Shephard wouldn't have the Gravity Gun, but the removal of the crossbow is keenly felt—especially when so much of the combat takes place in large, open rooms.

I don't think such open spaces are suited to Half-Life 2's combat at the best of time—the few times it's worked in Valve's own games have been incredibly specialised and highly polished encounters. Here, it's a brutal churn—the difficulty worse for feeling like survival is as much a matter of luck as skill. I had more success playing it safe, pumping round after round from my SMG into the Combine from distance. It was that or bite back a string of obscenities as I was taken down by yet another mob of soldiers.

Some of the visual design is interesting and distinct, although Nova Prospekt remains one of the blander locations of the Half-Life universe. The latter part of the game isn't much better, taking place in those Citadel style sci-fi corridors you'd seen enough of come the end of Episode One. There are a few, scattered moments that stand out, but they're few and far between.

There isn't much of a story, either. While Shephard is ostensibly set loose to assist Freeman, that plot thread is dropped pretty quickly. Instead, most of the story is told through flashback audio of Shephard's deployment to Black Mesa. Except, we already know the upshot of that: he goes to Black Mesa. The voice acting is, at least, competently done, even if little of what's heard is revelatory. The soundtrack, too, works well—setting up action and suspense that the design rarely capitalises on.

I went into Prospekt hoping for a fun, inconsequential trip through the Half-Life universe, interspersed with some twists to make it really stand out. That's what the best Half-Life 2 mods offer—campaigns like Minerva, Research and Development, or Mission Improbable. Even judged by that standard, Prospekt doesn't deliver. There are a few entertaining moments and neat ideas, but I'd grown dissatisfied and, worse, frustrated with it long before the end of its three hour running time. And if it doesn't impress on that level, it definitely doesn't work as a 8/$10 standalone game.

Is the absence of any notable Half-Life release like ants under your skin? Allow Prospekt, Valve-approved sequel to Gearbox's Half-Life: Opposing Force, to ease the long withdrawal. It's out today, priced 7.50/$10.

Perhaps you thought Freeman's run of good luck in fighting the Combine was a product of your own prodigious skill and a little intervention from the G-Man. You would be mistaken. Prospekt picks up as Freeman is marauding his way through Nova Prospekt prison. To keep the odds in his favour, the Vortigaunts teleport US Marine Adrian Shephard—you—into the fray.

Developer Richard Seabrook has been beavering away on Prospekt for two years, and claims it rivals Half-Life 2: Episode One for length. It's a standalone, so you won't need Half-Life 2 to play it, but of course that won't matter because you have it anyway. Assets from existing settings have been upgraded as befits a modern game and, most interesting, we're promised a return to the alien plane of Xen.