They are supposed to be two landmark installments of NBC's most successful cops-and-courts franchise.

Instead, two big episodes coming over the next week on "Law & Order" and spin-off hit "Law & Order: Special Victims Unit" stand like the yin and yang of the Peacock network's long-running institution: naked examples of how best to write off a cherished character and how not to shoehorn a guest star onto your series.

First the good news. Producers at "Law & Order" have given an impressive send-off to Jesse L. Martin's underrated Detective Ed Green, centering tonight's episode on an incident that recalls smooth operator Green's past as a gambling addict.

When news broke that Martin was leaving "L&O," headed for Jamie Foxx-level stardom playing Marvin Gaye in a highly anticipated biopic, I feared the worst. For a show centered on just-the-facts storytelling, the intrusion of farewell plot twists based on the characters' personal lives can feel as jarring as a 9mm slug in the back. (Remember that wack episode where pretty accessory Elisabeth Rohm finally made an impression as an assistant district attorney in her final episode, asking if she got fired because she was gay?)

But this is the new-model "L&O," stocked with crack character actors such as Jeremy Sisto as Green's partner, Cyrus Lupo, and Linus Roache as the new top dog assistant D.A., Michael Cutter.

So when Green winds up indicted for the shooting death of a former bookie, you know the end will come expertly, with a minimum of sentimentality and some long overdue showcase scenes for Martin and his longtime boss, S. Epatha Merkerson's Lt. Anita Van Buren.

(New guy Anthony Anderson's appearance as an internal affairs cop assigned to investigate Green's case is the cherry on this lovingly baked farewell cake.)

Now the bad news: The April 29 episode of "Law & Order: SVU" featuring guest actor Robin Williams is something else entirely.

I described it to a friend as "everything that's wrong with every 'Law & Order' franchise, wrapped in one pungent ball." And if anything, I worry that I underplayed the horror.

Here's the roll call: Superstar guest actor who viewers immediately know is the villain because it's the best part? Check. Absurd climactic ending featuring the two starring characters and the guest star? Check. Even more absurd last-act twist allowing the guest star to come back if he ever deigns to do TV again? Double check.

Williams is typically talented playing an evil prankster behind a ripped-from-the-headlines scam in which he convinces a fast-food manager he's a cop and gets him to sexually assault an employee.

That his nebbishy engineer could create an alibi by pretending to be different people talking to the cops on different phone lines I kept expecting one of the cops to blurt out "He sounds just like the guy from Mrs. Doubtfire!" is just one niggling problem.

By the episode's end, Williams' character has outfoxed Diane Neal's assistant D.A. Casey Novak in court while hoodwinking star detectives Olivia Benson (Mariska Hargitay) and Elliot Stabler (Christopher Meloni).

I can tell you this: I know which "Law & Order" franchise has earned my attention in weeks to come. And it isn't the one featuring a lukewarm redo of One Hour Photo. *