A couple get into a taxi cab. The husband notices the driver is going the wrong way. He asks the driver to stop. As the couple get out, the driver shoots them both. The SIU discovers the crime scene, and a homeless man who is an eyewitness to the crime.
There have been five victims so far - all robbed and killed, all found in remote locations. Interrogation of the homeless man Billy reveals little. The crimes occur at night, and start with calls for a cab with Allied Cab. An interrogation of a wanted felon and Allied cab driver Kaleed reveals that the fares are being intercepted by an unknown driver.
Another killing occurs, this time the victim's phone has fingerprints identifying Mark Lee Page. Page has a long rap sheet revealing a history of violence. Page used to run with a crew headed by Ted Madrid. Madrid reveals Page used to work as a collector for Madrid's finance company. Madrid gives Nash Page's address. Harvey and Antwon raid Page's apartment, but Page gets away.
Rachel discovers the doctored SIU files incriminating Nash. She confronts Lt. Ray Urbanski, but it is too late. Rachel confesses to Nash, as Urbanski makes his move - the SIU files are seized. Joe kicks Rachel out, giving her a transfer to the Sunset division.
Operation Flag Down begins, with the SIU posing as cab patrons to try to catch Page. Meanwhile Betty-Anne feels that she and Nash must mate in order to ensure the survival of the Serrabithian race. She goes to Nash's apartment with a gun. Nash distracts her, and Nick grabs Betty-Anne. She is arrested. Before she goes, she pats Joe on the back, forcing out a piece of apple that has been stuck in Joe's sinuses for an entire day.
Cassidy, along with Harvey, visit Evan's grave - a year has past since Evan's death. Driving along, Cassidy spots Page in his taxi. She follows the taxi to an alley. Page is waiting and takes her hostage. Cassidy is forced to drive the taxi - she crashes it into an abandoned school bus. A dazed Page still tries to get Cassidy, but she subdues him by slamming the taxi door on him.
Rachel breaks into Urbanski's office. She sends Nash (via e-mail) an incriminating file on what Urbanski has done to doctor the SIU files. But Urbanski returns to his office and catches Rachel. Rachel is arrested. The next day, Urbanski has been promoted to Deputy Chief. But Nash confronts Urbanski with the incriminating files, as the FBI arrest Urbanski.
Cassidy has decided to quit the police force. Nash and Nick meet her at the airport, where Cassidy has decided to go to Paris. Before she leaves, she gives Nash her badge.
For more, see commentary for Episode 122