OK, Kyle Katarn and Jan Orrs went after the plans on Danuta, where Kyle had to infiltrate an Imperial base (Dark Forces). It was a backup plan in case of failure when...
Princess Leia got the plans from Bria Tharen's Rebel cell on Topwara and transported them with presumed diplomatic immunity on the Tantive IV until Darth Vader decided to capture her (A New Hope). She sent the plans along with R2D2 to be delivered to Obi-Wan Kenobi.
Now it seems Garm Bel Ibis had arrangements to meet with a young Imperial turned Rebel spy on a backward rim world. Who knows for sure what data he was retrieving, but Isard failed to capture it.
Meanwhile, Vader was distracted by hunting down a surviving Jedi, the Dark Woman, and caught up with the game late, just in time to pursue Princess Leia.