“NCIS: Los Angeles” Season 7 returns after the midseason break with Episode 12 titled “Core Values.” The team investigates the radiation poisoning of a Marine Gunnery Sergeant, who has been moonlighting as a security guard at a nuclear power plant. [Warning: Spoiler Alert]

Sergeant Hugh Patterson suddenly gets sick, and collapses while on duty at a marine gun range. He is rushed to a hospital, where medical workers discover that he’s been exposed to dangerous levels of radiation. A thorough inspection of the range comes back clear, but the Sergeant’s pregnant wife reveals that her husband has been moonlighting as a security guard.

The NCIS team finds out that Patterson had not been moonlighting at a construction site, like he told his wife. It turns out that he has been working at Santa Flora Nuclear Power Plant, because the hourly pay is significantly higher than any other location.

Special agents G. Callen (Chris O’Donnell) and Sam Hanna (LL Cool J), visit the plant to investigate but they find it difficult to move freely. They are always being escorted around the property so Hetty (Linda Hunt) comes up with an idea. Kensi  Daniela Ruah) is assigned to go undercover as a janitor, while Deeks (Eric Christian Olsen) is assigned to take over the security guard position, that was vacated by the fallen Patterson.

After Kensi manages to get an email dump from the company’s servers, the team finds a suspicious email that mentions an incident that took place in the plant. Details are vague, but the incident was supposedly not reported because no one was harmed.

Meanwhile, Engineer Leo Chadmont, one of the plant employees, meets discreetly with Callen and Hanna. He tells them about his concerns about the safety of the facility. He thinks that the plant is a disaster waiting to happen and people should be made aware. Upon further investigation, the team finds out that the unreported incident involves the water that protects the radioactive fuel rods. It temporarily dropped down to a dangerous level about three weeks ago, but no one knew that Patterson had entered the containment room at that time.

Chadmont admits that he staged the accident, hoping that it will open the eyes of the necessary authorities. Unfortunately, he didn’t count on a vigilant security guard, who would check the premises at the time of his staged system malfunction.

With Chadmont under arrest, the team thinks that the case is closed. They get a massive surprise, when a backup generator in the plant explodes. They scour the area and find another explosive that has been planted among the water pipes. If the pipes explode, the plant’s cooling system will be compromised, causing a nuclear meltdown.

Callen and Hanna work together to defuse the bomb, and they manage to take it off the pipes and detonate at a safe distance. In the meantime, Kensi and Deeks see the surveillance videos and find a new suspect. They see a man named Rashad Nader (Ludwig Manukian), posing as a maintenance worker. Chadmont admits that he knows Nader, a man that he believes to be an academic doing research on nuclear power and its risks. He also confesses that he gave Nader access to the facility through a cloned I.D. card.

Callen and Hanna realize that the pipe bomb was only a distraction, when Chadmont tells them that it could not really have caused a nuclear disaster. They find another bomb at the bottom of the water reservoir, which could drain the entire containment pool in an instant.

This bomb is a lot more complicated, and has a remote cellular detonator. Kensi and Deeks try to jam cell signals in the area, to prevent remote detonation. Hetty calls off all emergency responders, in order to minimize casualties, in case the bomb goes off. This also buys them time, knowing that the perpetrators are waiting for the place to be crawling with people, before they detonate.

Kensi and Deeks manage to track down Nader, as he prepares to remotely detonate from a nearby coffee shop. However, before they could accost him, he manages to get to his cell phone. Luckily, Callen and Hanna have detached the blastcaps from the plastic explosives. Owen Granger jumps o the blast caps in a bomb squad suit, and manages to contain the explosion.

It’s another crazy day at the office for the team at “NCIS: Los Angeles.” Season 7 airs every Monday at 10:00 p.m. EST on CBS.