Casualty will mark its 1,000th episode with the return of actress Cathy Shipton for an adrenaline-charged instalment.
The milestone episode about Holby City Hospital's emergency department will be broadcast on Saturday on BBC One.
Cathy, who plays Lisa "Duffy" Duffin, featured in the original cast when the medical drama launched in 1986.
The London-born star spent 17 years in Casualty and has long been a favourite with viewers. She made a brief return for the show's 20th anniversary in 2006 before popping up again in 2015.
Ahead of her permanent return in August, the 1,000th episode will fill in more details about Duffy's life away from England.
Duffy will reconnect with her old colleague Charlie Fairhead (actor Derek Thompson who is also a member of the original cast), and reveal why she left her family in New Zealand.
Elsewhere in the episode, Dr Gardner (Jaye Griffiths) is given a hard time by colleagues who still resent her for dismissing Charlie. Dr Gardner's shift is made all the more challenging by a pregnant woman who refuses treatment which may be needed to save her baby.
A Saturday night stalwart for decades, Casualty has endured due to a combination of powerful writing and acting.
The first series consisted of just 14 episodes and a pilot. Thirty years on, Casualty runs for much of the year.
Memorable episodes include Closure from 1986 which revolved around a strike by the night shift to protest against closures.
Love Me Tender, first broadcast in 1998, featured a heart-rending scene in which Nurse Tina Seabrook, played by Claire Goose, admitted she was raped.
A poignant favourite is 2010 episode Nice And Easy Does It.
It depicted the assisted suicide of terminally-ill Nurse Megan Roach, portrayed by Brenda Fricker.
Brenda, an original Casualty cast member, won an Academy Award for her performance as Christy Brown's mother in My Left Foot, starring Daniel Day-Lewis as the Irish writer and painter.
She also played Macaulay Culkin's confidante in 1992 movie Home Alone 2: Lost In New York.