Anyone who completed Half-Life 2 - one of the most accomplished first-person shooters ever made - was left with more than a few niggling questions after the final credits rolled. Did you actually save City 17? Will hero Gordon Freeman escape from the G-Man? And did Alyx Vance, the game's leading lady, end up on the wrong end of a thermonuclear explosion?

We needed answers and, finally, we've got them. Half-Life 2: Episode One is the first in a trilogy of games that continue the story of Half-Life 2; Episode One kicks off seconds after that game's thrilling cliffhanger ending. But, despite obvious similarities, this is an entirely fresh adventure.

We won't give too much away, but Episode One immediately reveals how Gordon and Alyx survive the blast at the end of Half-Life 2. The Citadel is now on the verge of a second catastrophic explosion and your goal is to escape across the suburbs and catch a train out of the danger zone.

Episode One offers more of a breakneck potboiler in comparison to Half-Life 2's epic narrative. Five hours is enough to reach the end, but the five superbly-paced chapters are hugely satisfying, both in terms of plot and the sheer volume of action. It won't be long until you're pining for Episode Two - but this first episode never leaves you feeling short-changed.

Every effort is made to push Half-Life 2 forward and to avoid familiarity. Your surroundings are now actively involved in the gameplay. You begin by re-entering the Citadel in spectacular fashion and have to protect both yourself and Alyx from huge pieces of falling rubble with the super-physics gun. Puzzles and obstacles are even more believable as real-world challenges thanks to Episode One's robust and intuitive design.

It's Alyx who makes the biggest impact, though. The charismatic Miss Vance is always by your side and your involvement with her makes the game an incredibly absorbing experience. Alyx isn't invincible but she doesn't need to be wrapped in styrofoam peanuts either - mostly, she's putting you to shame with her gritty combat skills.