The ninth season of the drama begins with a bang, literally, when a large explosion kills an FDNY Captain. Mac and the team reluctantly allow an infamous arsonist named Leonard Brooks to be the arson investigator on the case, but are unsure if he is actually the culprit. Even after that case is solved, Brooks continues to taunt and torture the team when he poses as an elevator maintenance worker just before an apartment elevator combusts. In his personal life, Mac experiences flashbacks of his aphasia diagnosis, as he deals with frustrations that there is no easy cure. Jo, meanwhile, repairs her strained relationship with her daughter by being honest about a former flame from the FBI when he helps the team with a case. Other cases investigated by the CSIs include a dead body found on the Brooklyn Bridge that brings Flack and Boyd to San Francisco and a schoolteacher who stages a shooting at an election campaign, injuring Hamilton and Lindsay in the process.