The three episodes will finally shed light on the world of City 17 and the cataclysmic accident at Black Mesa. "Some stuff that's been a mystery from the beginning will be resolved, so you'll learn a lot about the characters and the history of the world. There's also going to be surprises," confirms Newell, with a sharp glint in his eye.

In an pleasing move, Episode One sees Alyx stay by your side for the entire adventure. Fortunately though, the developer has moved away from the clumsy team-play command system of Half-Life 2, which made organising your band of citizen warriors a headache.

Above: Alyx accompanies you through the whole of Episode One and she's a fantastic sidekick

"We spent a lot of time thinking about how you and Alyx could interact in ways that felt very natural and made sense, and at the same time were fairly deep," lead designer Robin Walker explains. "Trying to interact with characters with that sort of command-control, acting as if they were an RTS unit, was unnatural and sort of frustrating."

"It's the wrong direction to go in - it's like the cutscene of artificial intelligence," adds Newell. "We've developed the AI in Alyx's character enough that you can trust her, and depend on her, during the game. We've reached that point where putting a player into this co-op situation is fun and new, not frustrating and, like, 'I'm doomed now!'"

"Alyx has a really good understanding of what you and her are trying to achieve," points out Walker. "She makes really good tactical decisions. She's aware of what you're up to and she takes a lot of cues from you - if you flip over an Antlion with the grav gun, she's aware that it's causing problems and she'll take it out before anything else."