Jack and Kate discuss a situation in Marie's past. Meanwhile, Jack is still worried over the whereabouts of Kim; and Lynne draws the line about what she will and won’t take from a vocal Sherry Palmer.
A mail bomb that killed the wife of a nuclear physicist leads detectives to question the stability of the marriage. Forensics experts help Briscoe and Logan name a suspect, but Stone and Kincaid (Jill Hennessy) uncover a motive that no one suspected. Vincent Pastore (The Sopranos) guest stars.
Jack attempts to deliver the one person who can help to prevent the military attack now close at hand. Meanwhile, Palmer continues the push to prove the country is going to war based on false evidence and assigns his attorney to find out everything he can about Peter Kingsley, the man who hired the computer hacker.
Saunders tells Jack that the remaining 11 vials containing the virus are to be released within the hour. Meanwhile, Sherry makes a play to be Mrs. David Palmer once again.
Jack is faced with a horrible dilemma after he and Chase find the last vial containing the virus. Meanwhile, Tony could face the death penalty for putting Michelle's life before his duty to maintain national security.