Here's the BBC press release in all it gory detail:
"Richard Briers guest stars in this special episode of the medical drama, inspired by, and based on, the festive classic It's A Wonderful Life.
"With just minutes to go before Izzy's heart transplant operation, Elliot walks out of theatre in a daze at the day's preceding events – his son, James, has been sentenced to prison; his daughter, Martha, has been badly injured from the shootings; and Zoe has been shot dead by Tim, who has also taken his own life.
"Feeling unable to go on, Elliot walks towards a bridge and prepares to jump into the icy water below. A man called George interrupts him, however, and asks him to help a woman who is just about to give birth. Elliot wishes he was never born, and, in true It's A Wonderful Life fashion, his wish is granted.
"Viewers then witness the world without Elliot. Joseph is a patient in a psychiatric ward, Darwin has been shut down following the shootings, Connie doesn't recognise him, Mark is in a wheelchair and many more of the staff have died as a result of the shootings.”
Bet you can’t wait...