It's single-player HL2 mod that displays the author's exceptional grasp of the Source Engine and level design. Its tone is distinctly "Half-Lifey," though its narrative is decidedly old school. All of your direct information is delivered to you via on-screen text transmission, thanks to whoever's pulling the strings. (you'll find out) While it uses HL2 content for its content and look, it will feel both foreign and familiar. Sure, you've seen Combine tech and Manhacks before, but the design of the island's full base is pretty impressive. Throw in a community-made soundtrack that smacks of Valve quality, and you have the best HL2 mod you've never played.

IIRC, it clocks in around 3 hours for those who plow through in a single sitting. Maybe longer, though... it's been a while.