Sentences are differently build up in German so excuse the crappy translation.
day 14 is a single-player map of different Art, thenm you change the story of the original game. Do you remember the (stelle ziemlich) from the beginning of Half-Life 2, at the point were gorden first meets Alyx and gets saved?
Now assume he didnt get saved by Alyx. Gordon becomes transported (statdessen) in a (outpost here i think) from the Combine, where he is medicaly researched in 13 days. he escapes at day 14. The cell door is open and dead combines are spread around everywhere, a suspecious looking man is disappearing beyond the corner. Then it creeps up to you, is the newly gained freedom from short duration? Is it all a test from Aliens, whom sees a inteligence resource in Gordon? the tension builds up, and Day 14 reminds a lot of games like Myst: The arcitecture is surreal, and gordon needs to solve small riddels to get his gravity gun.
Alright, this should be roughly it, though German nor English is my native language so please bear with me.