Shuttle Underground fighters was a mission in Jak II. After Jak defeated the Baron at the Palace, he returned to Torn, who reported Haven City was on high alert with the Krimzon Guard looking for Jak and Daxter. Torn needed Jak to move four Underground members to new safehouses due to the increased KG activity.
A sort of racing annex escort mission. You'll have to use a two-seated or three-seated zoomer and drive around picking up members of the Underground, then drop them off at a new location. There's a time limit for picking up a member (one minute), with an added minute when you actually pick them up. The timer then resets to one minute when you drop him off again.
Since the Krimzon Guard will be onto you soon enough, just stay on the lower level. It's otherwise pretty much reliant on your driving skills to get around fast enough while avoiding the KG patrols and guards. All members are picked up and dropped off in the Slums district, except for the last one which you need to drop off at the Water Slums.