NCIS executive producer Shane Brennan won't know whether his as-yet-untitled spinoff is a go until May 20, when CBS announces its fall schedule. In the meantime, he gave us an inside look at the April 28 and May 5 episodes of NCIS, which introduce the characters that would be at the center of the sister series. The two-parter ''Legend'' finds McGee (Sean Murray) and Gibbs (Mark Harmon) heading to Los Angeles to track a terrorist cell in a joint investigation with the Naval Criminal Investigative Service Office of Special Projects. Its agents, led by LL Cool J and Chris O'Donnell (pictured, filming), specialize in undercover ops. Read on for full character breakdowns of the would-be spinoff cast, more photos from the episodes (LL does his own stunts!), and, of course, a ''Tiva'' tease...