Lt. Barclay is part of something called 'Project Pathfinder', which is charged with bringing home Voyager from the Delta Quadrant. The project idea is to open a small artificial wornhole near Voyager and hold it open long enough for the ship to see it and come through. But the project leaders want to know how the crew will react to the opportunity , and how they might present it to maximize the probability that Voyager will take it. So Barclay (now married) sets up a holodeck simulation of the Voyager crew, running through various permutations to test their viability. As he does, he becomes obsessed with them, regressing back into his holo-addiction. The holo-addiction takes over Barclay's life, threatening his marriage, etc. and Counselor Troi is brought in to snap him out of it for the second time.
