The combat explodes across this soothing canvas with a brilliantly messy splat. Something clever involving particle physics has allowed Valve to make thick black blood, lurid yellow goo and something a lot like vomit spray repulsively from your victims with every cracking impact. The new poisonous Worker Antlions burst like bioluminescent bombs; injured Hunters drool a sticky slurry of their own innards from where their mouth should be; and when the vortigaunts fight... Jesus God. The trailers released last year showed nothing of this - some consolation for those of us who spoiled big chunks of the game for ourselves by watching them.

The three-legged Hunter creatures are the highlight of the fighting: Velociraptors to the Strider's T-Rex. They're the perfect size for Gordon-killing: compact enough to chase you indoors but hefty enough to take the shotgun blast that awaits them there. More importantly, they're bright enough to do so when you least expect it. Valve have trained them to deduce where you're heading and get there first by a different route, and the effect is alarming.

The sense of threat is a prevailing and escalating theme of Episode Two, and it extends to the plot. You and your friends are trapped, maimed and violated in ways that are distressing on a really visceral level, and it's properly gruesome to watch. The blood-soaked tone gives the story a force that makes it the darkest and most exhilarating chapter yet. It's Half-Life's Empire Strikes Back - and it even has a less snowy analogue to the Battle of Hoth.