"If you're a fan of ER, you won't want to miss this one," the exec producer tells PEOPLE

Credit: Lionel Hahn/Abaca; Bruce Glikas/FilmMagic

George Clooney will be back in his ER scrubs for one final episode of the NBC medical drama that will air March 12.

A show insider tells PEOPLE that Clooney will reunite with Julianna Margulies in a storyline that also features Susan Sarandon, making her ER debut as a grandmother grief-stricken over the sudden death of her grandson.

Although execs of Warner Brothers and NBC have been coy about Clooney’s return, ER‘s executive producer John Wells did tell PEOPLE on Wednesday that the March 12 episode will be a memorable one.

“If you’re a fan of ER, you won’t want to miss this one,” said Wells, stopping short of naming the main attraction, Clooney as Dr. Doug Ross.

Reunited Lovers

The episode brings Clooney’s character back to his longtime love Margulies’ nurse Carol Hathaway. Will the star-crossed pair finally get their happy ending?

Wells, who wrote the series’ final episodes, would say only, “I want their story to be very fulfilling in the end.”

Clooney, 47, left ER in 1999 for roles on the big screen. Margulies, 42, who exited the series the following year, recently welcomed her first child Kieran, with husband, attorney Keith Lieberthal.

The show began shooting its two-hour final episode this week. Its series finale will air April 2.