On the latest "NCIS: Los Angeles," a murder at a rail yard unleashed an unexpected chain of events. 

On "The Livelong Day," a security guard at a rail yard is killed when a man in a mask tosses him onto the tracks in the path of an oncoming train. A railway break man's test gauge with the name "Rome" on it leads Sam (LL Cool J) and Callen (Chris O'Donnell) to Mitchell Rome, who was fired after he tested positive for marijuana and caused a collision in the yard. His roommate, Gil Bellamy, is an unemployed demolitions expert. Sam and Callen believe they have their culprits.

Callen and Sam almost get hurt in an explosion when checking out Bellamy's hotel room. Kensi (Daniela Ruah) and Deeks (Eric Christian Olsen) run into a private investigator, Trager, who says he was hired by the train company to find dirt on Mitch.

Deeks and Kensi then go undercover to investigate the train company's files, and set up their own explosion to clear the building to give them time to download files, grab shredded papers and clear out filing cabinets.

They then have to figure out if Mitch is innocent, as his mother says, and if Bellamy is the real culprit.

They later find out that Bellamy spurred Mitch into action. Mitch thought that Bellamy was doing a public service, and that he wanted the public to know that hazardous materials were being transported through high threat urban areas, which is a violation of federal standards. But Mitch doesn't know that Bellamy is working with Traeger and the train company, and that he set up a bomb along the track.

The team goes to the track. As Mitch approaches the bomb, he's unable to slow down, but Sam finds the C4 and receiver at the last minute. Everyone comes out of the ordeal unscathed, and Trager is arrested.

On the next episode, "Fallout," a dirty bomb could jeopardize L.A.'s existence. 

The synopsis reads: "When an antiterrorism device is stolen, the team must quickly find the suspect and the device before an attack is made against Los Angeles. Meanwhile, Hetty realizes the case is bringing her back to someone from her past."

Watch the promo for "Fallout" below, which airs Tuesday at 9 p.m. on CBS.