My copy is out on loan at the moment, but I can give you a general summary.
Minor spoilers to follow:
The basic set-up for the story is that the Triskelion is invaded by a group of people in white and green outfits, who take out a couple of security details before Tony Stark suits up as Iron Man and confronts them. He convinces them to sit down and talk. The rest of the Ultimates are called in, and the invaders, who are dubbed "The Tomorrow Men", reveal that they're from the future. They live in a world where an organization called Tiber basically runs everything and subjugates everyone. The Tomorrow Men (who include a couple of women, if I recall correctly), say that they've come back in time to track down the origins of Tiber, which they've traced back to this point in the 21st century.
Jan gets to do some undercover work in tracking a Tiber agent to his headquarters, and even Hank Pym is called in to play a part. The Ultimates find and invade the first of three bases used by Tiber. However, while in the depths of this base, Tony Stark discovers a rather nasty surprise, and ends up dealing with Tiber in a time and place he didn't expect.
I don't want to give too much away from this point on, in case you decide to read this one for yourself. The author appears to have read his source material (including some Norse mythology), and he seems to have written this with the Ultimates specifically in mind, although at the time he wrote it, much of Ultimates2 hadn't been published yet, so some of the characters may seem a little off.