They say that we only use 20% of our brains. That being the case, butt-chinned Michael Jennings reasons, it should be fine to let an excitable leprechaun blast at least 80% of one's brain with lasers. And so this is how our hero makes his fortune. By reverse engineering pointless new technologies for rival companies and subsequently allowing Paul Giamatti (that weird guy that always manages to convince you that you don't hate his performance for some reason) to practice his Star Fox skillz on incriminating memory cells, Jennings essentially trades his memory of his dubious business practices for a…PAYCHECK! He's paid in the form of PAYCHECKS, you see.
During an attempt to aquire such a check, Michael falls in love with Uma Thurman (reprising her role as run-down prostitute Fantine), who presumably also collects regular PAYCHECKS. Yes, Mike's got it all: a job working for billionaire Jimmy Rethrick, his old pal from Butt Chin U; a tepid relationship that he's guaranteed to forget forever; and of course the promise of that most desirable and titular of checks, the PAYCHECK.
But Rethrick's been making a lot of transparently evil faces in scenes without our hero, and the FBI is on Michael's trail for whatever he's been working on. You see, FBI agents also receive paychecks, but they earn them by CATCHING MICHAEL JENNINGS AT ALL COSTS!!!