The plot sees the arrival of Professor John Gaskell (Paul McGann) at Holby City Hospital, shortly before Fredrik Johanssen (Billy Postlethwaite) goes on a shooting spree, which results in the death of Raf di Lucca (Joe McFadden).

Holby City executive producer Simon Harper first teased the two-part storyline in November 2017, saying the characters would face the "hospital's greatest peril yet" and their lives would be changed forever.

The staff of Holby City Hospital prepare for the arrival of renowned surgeon Professor John Gaskell (Paul McGann), who has used stem cell treatment to reverse motor neuron disease (MND) in a patient.

Gaskell brings in a patient with a spindle cell lipoma and he asks Jac Naylor (Rosie Marcel) to join him in surgery.

Feeling under pressure to prove herself, Jac removes Oliver Valentine (James Anderson) from his scheduled operation so she can perform it herself, using the opportunity as practice.

On AAU, Essie Harrison (Kaye Wragg) expresses her fears to her husband Raf di Lucca (Joe McFadden) that something bad is going to happen when she notices that she has lost her wedding ring.

Nurse Donna Jackson (Jaye Jacobs) shows Raf some threatening text messages from Jeremy Warren (Nick Rhys), who blames the hospital for his mother's death.

Afterwards, Jac breaks down in Adrian Fletcher's (Alex Walkinshaw) arms, as her grief over her sister's death and other recent events catch up with her.

In the wet lab, Gaskell introduces his MND patient, László Furz, to Essie, Henrik Hanssen (Guy Henry) and Sacha Levy (Bob Barrett).

She gives him her resignation letter to hand to Hanssen, but as she makes her way down the stairs, she is greeted by an anonymous figure, who shoots her and leaves her to die.

Paramedic Iain Dean (Michael Stevenson) is handed Raf's patient, who is found with a gunshot wound to her leg, and he treats her in an ambulance outside.

He makes it onto Keller ward, where he intends to shoot Dominic Copeland (David Ames), but Hanssen interrupts and tries to talk with his son.

On 21 November 2017, it was announced that Holby City would be airing a two-part episode the following month, that would result in the hospital being "changed forever".

[1] Sophie Dainty of Digital Spy confirmed that Paul McGann's character Professor John Gaskell would be introduced, while series regulars Jac Naylor (Rosie Marcel), Ric Griffin (Hugh Quarshie) and Henrik Hanssen (Guy Henry) would feature prominently.

[1] The show's executive producer, Simon Harper, stated "You'll see our NHS heroes instinctively rush towards the hospital's greatest peril yet in a story that changes lives forever – and the fact that the episodes introduce Paul, one of our country's most internationally renowned screen talents, is the icing on the cake.

[4] McGann's casting was announced in April 2017, and Dainty (Digital Spy) reported that his character would be "the saviour of the hour" when he made his debut in December amid a crisis at the hospital.

[6] During an interview with Allison Jones of Inside Soap, McGann pointed out that "Group Animal" was a departure from the show's regular plots, and it was something the producers had thought long and hard about beforehand.

[4] The episodes also feature an appearance from Iain Dean (Michael Stevenson), a paramedic from Holby City's sister show Casualty.

[12] "Group Animal" was first teased in a September 2017 trailer called "The Reel Before The Storm", which showcased the upcoming Autumn storylines.

This was the highest rated episode of Holby City since "The Price We Pay", which aired on 28 February 2017 and attracted 5.34 million viewers.

[24] The "Group Animal" storyline contributed to Holby City's nomination in the Soap and Continuing Drama category at the Royal Television Society Awards in March 2018.