This mission is given to you by the FIB ("B" marker on your map) and can be started by either Michael or Trevor. Steve Haines wants you to investigate a research lab that seems to be developing nerve gas, although he mentions that you're going to need a bunch of things, one of which will cost $2 million.
Since you're all pretty broke, you're going to need some help with this one, so Michael will call Lester while both of them drive to a bank in the Paleto Bay area. When you finally get there and Lester as well, go to the bank and do some surveillance in the front and the back. Shoot the alarm in the bank, then get to the gas station across the street to monitor the response time.
After some time, Trevor gets out of the vehicle and hops on a motorcycle. You can choose either Michael or Trevor here to race back to Trevor's offices. You don't get anything, other than fulfilling a challenge, for winning the race. Once you get back, watch the cutscene and then choose your gunman for the job (you most likely have Chef as a choice now). The mission will end after another short scene.