Fans online “celebrated” the 10-year anniversary by cutting together a video of Valve’s co-founder and managing director Gabe Newell talking about his company’s plans for a Half-Life sequel over the years. “We know how the trilogy ends and there’s a bunch of loose ends and narrative arcs that need to come to a conclusion in Episode Three,” he said in August 2007. “I don’t have anything to say ... but yes, of course we’re doing Episode Three,” was the line in September 2009. “I got nothing to say about Half-Life,” he said in August 2011. “I don’t know this man at all,” he joked in March 2013, when the British host Jonathan Ross begged him for further news of a sequel.

Why do people still care so much? The original Half-Life, released for the PC in 1998, remains a definitive work in “first-person” gaming, a sci-fi thriller that set new standards for storytelling techniques and immersive world-building in a genre that had always boiled down to guns and blood. Previous first-person shooters like Doom and Quake embodied simplicity: Walk into room, kill monsters with weapons, rinse, and repeat.

Half-Life began with the player’s avatar, the scientist Gordon Freeman, ambling through a massive underground lab on a normal workday before a terrible accident flooded it with aliens. Valve embraced the limitation of the first-person perspective (players can only see through Gordon’s eyes) by having a huge story unfold around him in tiny bits and pieces: Players could put together what was happening if they paid attention to overheard bits of dialogue, or watched other characters interact from afar. The game was in every way a revolution, and remains a wonderfully scary, inventive work almost 20 years after its release.

Valve

Valve tinkered with Half-Life over the years, offering add-on games that shifted the viewer’s perspective (in Blue Shift, you played as a security guard watching the crisis unfold; in Opposing Force, you jumped into the skin of one of the original game’s villains). Finally, they gave fans a full sequel in 2004 with Half-Life 2, which is still generally regarded as the greatest PC game ever released. The world of Half-Life 2 wasn’t an underground lab but a dystopic Earth conquered by a mysterious extra-terrestrial militia, and the game navigated through diverse environments (vast cities, abandoned sewers, a haunted village, an alien prison) and drew from every genre imaginable. Half-Life 2 could be a first-person shooter at one minute, a racing game the next, then a grim work of horror, then a goofy alien adventure that saw the player commanding hordes of giant bugs against the enemy.

Half-Life 2 is the kind of game that’s impossible to overhype: Even played now, when its technological advancements seem routine, it remains better than almost any contemporary first-person game. Its use of physics, in which every object can be lifted and thrown and many used to solve puzzles, changed the extent to which a game’s environment could feel like a real place the player could interact with. Before Half-Life 2, the worlds of many games were little more than colorful backgrounds to move through; Valve helped make them infinitely more immersive. Half-Life 2 also gave the player a companion, Alyx Vance, who helped at various points throughout the game. These computer-controlled partners had existed in games for years, but usually as incompetent robots who’d only get in the player’s way; meanwhile Alyx felt like a real character the player could rely on.