How and Where/Who and Why (LOD/Hartman crossover)

"How and Where"/"Who and Why" are a two-part crossover event between the two series Law & Order: Dallas and Hartman (a Law & Order: New Orleans spin-off). "How and Where" is the first part that is seen in Dallas Season 10 episode 9. "Who and Why" is the concluding part that is seen in Hartman Season 7 episode 9.  

In the crossover event, a murdered drug smuggler will lead the Dallas detectives to a person of interest, whose an former Los Angeles detective now working for a federal task force deep cover. However, when that agent is missing and his life on the line. The detectives must seek help from the agent's former co-workers, led by Sergeant Hartman, and as well the task force. But the case ends up being much more complicated as the never-ending revelations include a infamous drug cartel, crooked senators, and a mole deep in the LAPD.

"How and Where" (L&O: Dallas)
Diaz and Mathis probe a murder of a drug smuggler, Daniel Jimenez, in a stage car accident. Their investigation and help from DEA Agent Neil Arturo (Nestor Carbonell) leads them to the dangerous Marinosa cartel. However, in order to smoke out the key members, they need to find more evidence. And soon the investigation reveals to them about another smuggler being with Jimenez hours before he died. And with Arturo's assistance discover the other smuggler is a up-and-comer, Ross Glen (Steven Pasquale). However, the case takes a surprising turn when their person-of-interest is nowhere to be seen and that he has been in deep cover for a federal task force. Glen is actually Glenn Rossum, a former Los Angeles detective. With nothing on him in Dallas, Diaz and Mathis seek help from Rossum's old boss, Sergeant Chloe Hartman (Jessica Chastain). Hartman and as well her fellow Detectives Jesse Temple (Michael Ealy) and Beth Greico (Beau Garrett) arrive to Dallas to assist in finding their friend and try and link the Marinosas to the murder and Rossum's disappearance. However impediments by the federal task force and Inspector General Markey (Burke) could lead the Dallas and LA's joint investigation into jeopardy.

"Who and Why" (Hartman)
Hartman, Temple, and Beth return to Los Angeles from Dallas following the murder of Detective Glenn Rossum (Steven Pasquale) to find out who in the LAPD sold him out to the Marinosa cartel. After no promising leads turn up, Dallas P.D. Detectives Bianca Diaz (Dania Ramirez) and Trey Mathis (Paul Telfer) and DEA Agent Neil Arturo (Nestor Carbonell) and other members of the task force arrive to assist.

Shortly after they arrive, Hartman's son Jack and Corporal Tim Rook are ambushed by men with automatic weapons, though both manage to survive. Realizing that the Marinosa cartel was sending a message to them, the Major Case Squad with the help of the Dallas detectives and the task force turn up the search for Benito Marinosa and the mole, unaware that the latter is much closer than they think...

Law & Order: Dallas

 * Dania Ramirez as Senior Detecitve Bianca Diaz
 * Paul Telfer as Junior Detective Trey Mathis
 * Terry O'Quinn as Homicide Unit Captain Eric Dalton
 * Jay Hernandez as Senior ADA Carlos Gutierrez
 * Elena Satine as Junior ADA Darlene Scott

Hartman

 * Jessica Chastain as Sergeant Chloe Hartman
 * Michael Ealy as Senior Detective Jesse Temple
 * Beau Garrett as Junior Detective Beth Greico
 * Jonathan Togo as Corporal Tim Rook
 * Joel Courtney as Officer Jack Hartman
 * Ricardo Antonio Chavira as Lieutenant Carlos Mendoza
 * Viggo Mortenson as Captain Paul Thorne
 * Terrence Howard as Deputy District Attorney Jonah Dekker
 * Tia Mowry-Hardrict as Deputy District Attorney Cara Winston
 * Geena Davis as District Attorney Ellis Fontana

L&O: Dallas

 * Jon Tenney as M.E. Dr. Augustus Simon
 * Richard Schiff as Dr. Elliot Harvey
 * Glenn Close as District Attorney Melanie Carson
 * Omar Benson Miller as DA Investigator Lou Finn

Hartman

 * Tim Daly as M.E. Tucker Monroe
 * Mouna Traoré as Forensic Specialist Kendra Hastings
 * Kevin Zegers as Tech Analyst Jasper Baron
 * Brenda Song as Officer Sunny Ng
 * Sela Ward as Sarah Thorne
 * Rick Hoffman as I.A. Sergeant Dick Lockridge

Guest Cast

 * Alec Baldwin as DEA Agent Harrison Drayton
 * Nestor Carbonell as DEA Agent Neil Arturo
 * Steven Pasquale as Former LAPD Detective Glenn Rossum/Ross Glen
 * Vincent Laresca as Benito Marinosa
 * Billy Burke as Dallas Inspector General Markey