A bomb goes off in a nightclub, and the perpetrator is killed in the process. The team examine the body, and discover traces of radioactive material, sparking suspicion that another device has been planted somewhere in the city. The case takes an unexpected turn when it transpires the bomber was stabbed before the explosion - and that Horatio's supposedly dead brother could have been involved. Meanwhile, Calleigh fears she can no longer remain a CSI after a man holds a gun to her head while she was examining a crime scene.