You know how time travel movies are. A small group of researchers, funded by a mysterious Mr. Landau, built themselves a time machine. They send one guy, Jim, forward one hour, he finds dead bodies of the team, and one of them is holding a camera he was using to record everything. Jim caught a brief glimpse of someone dragging Gail, who also works on the project and is his girlfriend into the only elevator out, but the place is set to self-destruct, so Jim goes back to the time he originally came from to convince everyone they must avert this horrible fate. Their time machine drew so much power it blew out all the power in the city, so the elevator isn't working, so that's out. And the NSA, suspicious of Mr. Landau, is upstairs trying to get to them anyway, to rescue their agent.
So there's a lot of arguing about whether they could go further back and avert the whole thing, or whether their fates are set. They watch what's recorded on the camera, but seem to keep doing a lot of it. At one point, they see that Jim and Gail successfully found Landau in the server room, so they go ahead and do that. Except finding Landau obviously didn't save them last time, so shouldn't they be trying to do different things? Easy for me to say, sitting comfortably in my living room.
I didn't have a sense of how large the underground facility they were in was. There were at least a half-dozen rooms and a couple of halls, probably more, but I started to wonder how the killer kept successfully getting away from them with a limited amount of places to hide.
I have to give the film some credit, I was plausibly able to construct scenarios for basically every character to be the killer. Greed, jealousy, past mistakes, ambition, simply believing it was fated. But you can rule out several of them just based on the killer's size in the scenes where they interact with other characters (the killer is wearing full tactical gear and a gas mask). Some of the characters are simply too big to be the killer, but beyond that, everyone was hiding something, or had some motivation that could explain it. So that was something.