What if everything that went wrong on "A Red Letter Day" was not caused by what you thought? What if your teleport ripped a hole in space and time? Now you must travel through time to correct the events and set right your past. Causality Effect A Red Letter Day Gone Wrong
As a couple others have noted, some bits seemed pointless, such as the gunship you can't shoot down. And unless I had read the other comments I wouldn't have had any idea that Breen & Alyx were on the roof in the last scene, let alone that I should try to kill them -- her anyway. I'm not sure if there was a glitch or if I just didn't find the exact trigger spot, but I had a hell of a time getting out of the first underground office, where you use the gravity gun to blast through the wall -- that was a clever bit! But that dam door would NOT open for the longest time, and I wasted hours trying to get up the ladder, blow up another pipe, retrace my steps to another door, etc. maybe it's just me. Someone else complained about the antlion guard -- I thought he was pretty easy. Rolled a grenade out on the walkway where it blew up the two barrels (of course duck back inside) and then 3 or 4 crossbow bolts put him down. I also did it just with the crossbow, but that took up nearly all the bolts, and left me with just 2 I think to take out the goons who rappel in for the next scene around the left corner.
All in all, a very nice mod though, and interesting concept even if it's a little uncertain what's going on. Is it a parallel universe? An alternate timeline? I got to a 2nd ending where Kleiner lives, but how can that be when we see him shot right at the beginning? Alyx's last "He doesn't have a clue, does he?" really kind of summed it up; it was almost like dropping in 3/4 of the way through the second episode of a 2-part Dr. Who story.
Very good idea though, as I say, and hope to see more. I would REALLY like to see a mod that isn't so linear, like all of HL is. Definite cusp moments leading to completely different maps and such, and a few distinctly different endings -- maybe 2 ways to "win", 1 to lose, 1 to draw? Good luck and I look forward to your next.
Fun action and different endings mean plenty of replay-value (Or at the least, playing from a previous save) on top of great level-design and scripted sequences that tell a good story make this mod a must play, so come on up and give Causality effect a whirl. :)