By keeping you fighting alongside Alyx, Episode One has a hectic and different flavour to Half-Life 2. "Episode One led us to more interesting combat," says Walker. "Not just 'kill everything in sight' but another level of objectives you need to be completing while dealing with other threats. So, thank God you've got Alyx, because she can help you through that."

Above: Episode One is out Thursday. Episode Two will launch at Christmas and Episode Three will be released next year

Valve is justifiably pleased with the way in which Alyx works alongside you but never burdens you. "We can do the usual sort of escort experience but reverse the responsibility a couple times," adds Walker. "So there's a time where Alyx has the only gun but she can't see anything without your flashlight. She's relying on you completely at the same time that you're relying on her. We think it's definitely something gamers have never experienced before. They've never had to play that kind of role before, whether with a friend or NPC."

Our Half-Life 2: Episode One coverage continues tomorrow and we'll have the full review for you on Thursday. Don't go anywhere else.