The team investigates the murder of three co-defendants, who were out individually having some last minute fun on the eve of their trial for drug-related homicide. The likely suspect is a rival drug dealer, but unique taser burn marks on one of the victims leads Mac to an eye-witness he had made a promise to a year ago. She was a reluctant witness in the case against three dead vics, so Mac convinced her to tell him what she knew and promised he'd find evidence to keep her off the stand. But when key evidence was suppressed and the eyewitness was subpoenaed to testify, our three vics tracked her down and slashed her face as a warning to keep her mouth shut. While she has the motive, she couldn't have been at all three places at once, leading CSI to look into who sought justice on her behalf.