The BBC suspended production in March, along with EastEnders and Holby City, as Covid-19 spread across the UK.
The new episode “will examine the personal cost of being national heroes at a time of crisis”, according to the broadcaster.
Social distancing is being maintained on set, with cast and crew using “inventive techniques and careful planning” to maintain production standards.
The long-running programme will air at a later date with shorter, 40-minute episodes.
Clinical lead Connie Beauchamp (Amanda Mealing) will have her leadership qualities pushed to the limit as several members of her team fall ill with the virus.
She will also be challenged to keep her personal feelings in check when her lover Jacob (Charles Venn) tests positive.
Connie and senior staff member Charlie (Derek Thompson) will together face their “most gut-wrenching decision ever”.
The pandemic and its aftermath will also put pressure on the general staff.
Paramedic Lev (Uriel Emil) will decide to move out of the family home during lockdown to protect his vulnerable young son who is recovering from cancer.
Without her husband at home, Faith (Kirsty Mitchell) must come to terms with Dylan’s (Will Beck) accusation that Lev had an affair.
Kate Oates, head of continuing drama at BBC Studios, said: “We are thrilled to be back in production with BBC’s longest-running medical show.
“The writers, Casualty teams, cast and crew are back on set with stories that reflect the extraordinary times we are living through.
“We can’t wait for the audience to see them and to be back in the Saturday night schedule.”
