It is a beautiful day in New York City, but what the people in New York don't know is that bomb is about to go off. Someone is inside a building, busy getting a bomb ready to go off from a cell phone. Detective Flack and Lindsay are informed about a security guard that missed his scheduled check in.
The blood trail that Mack finds leads to the bomb that is hidden above the ceiling panels in the building. The trail also leads to why a guard was killed. Mack phones Lindsay about the bomb, and she gets everyone outside away from the building as fast as she can before the bomb goes off. Mack and Don however are delayed with a straggling civilian and are still inside when the bomb explodes.
Lindsay, Stella, and Danny frantically wonder if Mack and Don survived the blast. Stella feels confident that Mack did because he is a marine.
Stella gets help from the FBI and the Department of Homeland Security, who have special forensic capabilities to help solve the case. When Stella is meeting with the DHS rep, she meets a man named Dean Lessing who recognizes her from when they both attended a certain seminar on bomb physics.
Mack regains consciousness and finds the civilian in good health. Mack asks the man about a blue powder surrounding them, and he explains that a copier machine used to be on the floor above their position and that it must have fallen through. As Mack finds Don covered in debris, he immediately tries to save his friend's life by trying to stop the serious blood flow inside Flack's open chest wound. As he does this, he flashes back to 1983, where he is in Beirut telling an similarly injured Marine to hang on. The civilian assists Mack and is amazed that he knows what to do.
After Mack is rescued, Stella begins updating him on what they're currently doing with the evidence. She also tells them that they will soon start getting images together to see if they can find any more bombs. Mack is worried about Detective Flack and whether he will be okay. Stella also notices not only his current injuries, but an old one that she's never seen before.
After the pictures are sent to the rest of the team, Danny finds out that the bombs are made with C-4, which is a military grade explosive. Sid tells them that the security guard was killed with military technique, which means that the security guard was killed with a knife that twisted the aortic wall.
After Detective Flack has his operation, Mack has the unpleasant task of taking pictures of his friend as a part of the crime scene. Mack is determined to get his friend through this tragedy.
After asking Lindsay about her condition (as she had been slightly injured by blast), Lindsay shows Mack what the bomb simulation looks like on a computer with sound included. The bomb stimulation also shows everyone's positions. Dr. Hawkes is a bit frustrated because everybody went through that crime scene with a fine tooth comb and still can't find the trigger mechanism that set the bomb off. Mack says that it may have ended up as the shrapnel that caused Flack's chest wound.
They get evidence that came from Detective Flack's chest during the operation. Mack, Lindsay and Dr. Hawks all hope that the trigger is contained within that debris. After they discover that the debris is a cellphone that was used to remotely recieve a number to set the bomb off, Mack Taylor gets some very strange calls. The bomber indicates that another bomb will be set off in 1600 hours. That will give the team three hours to find it before it goes off.
To get Danny and Lindsay an idea of how to find the exact number that triggered the bomb, they remove the SIM card from the bomb's cellphone that stores cellphone numbers as well as who called them. This particular card contains a number that goes into a reader which gives them all the information, such as to whether the cellphone has restricted access. What they find with this one is that the number has restricted access: the DHS rep's cellphone.
Mack believes that someone from the DHS may have triggered the bomb to go off. The DHS rep tells him them that government laptops were stolen two weeks ago, and they would contain certain numbers and codes (such as responding to a pager) that DHS agents are required to respond to. The Labor suggests they should down secure lines to see if they could stop the bomber that way. Mack retorts that the secure lines aren't the problem, it's the cellular network; you don't need a secure line to respond to a page: any cellphone in New York could trigger the bomb.
Mack gets a what turns out to be a third pager call from the bomber to try to trick him to set off the bomb. He looks at his cellphone he sees a particular date, 10-23-83, on his cell phone. He explains to Stella that it's the date of the 1983 Beirut barracks bombing, of which he witnessed and was injured in. Mack takes the bomber's next cell call, but they are unable to get enough time to trace it. Mack however realizes that they can now override the bomber's cellphone and turns it back on.
The signal leads them to the garage of an abandoned house. Inside, Mack and Stella are accompanied by Dean Lessing and find the bomber's workshop; one of the government laptops, photos of potential target buildings, several automatic weapons (with an assault rifle notably missing) and Mack's complete military service record. Mack finds reports of the bomber's attempts to inlist with the Marines three times, but he couldn't pass the psychological requirements. The bomber calls Mack, commending the idea of overriding of his cellphone, but tells Mack the he is simply a part of the bomber's demonstration and not the intended target. In the meantime, the bomb's cellphone to traced to Chelsea University.
Along with Danny Messer and several members of the Bomb Squad, they find the bomb, cellphone and all, in the library of Chelsea University. Mack and Danny suit up in bomb squad gear, and Mack cuts the wires to separate the cellphone so that the bomb doesn't go off.
They believe the bomber could be schizophrenic and very dangerous because he stopped taking his medication.
Mack Taylor asks for backup in looking for Dean Lessing who is at the library and tells everybody that there are more bombs set to go off.
In order to get the bomber to let go of his weapon, Mack gives the bomber a chance to do what he has always wanted to do: be a marine. Mack uses his rank as a superior officer in Marines to get the bomber to put down his weapon.
While they await word on Detective Flack's condition, Stella congratulates him on the way he handled the bomber and how no one got hurt. Mack then tells Stella about an important day in Beirut when a corporal died it in his arms.
Mack believes that the bomber didn't really want to kill anybody, but that he whated to prove that he was able to be a marine despite his rejection letters.
Mack goes in for a private moment with Detective Flack and asks him to squeeze his hand for reassurance that he will be fine.