MDA is an Australian Medical Defense Organisation. It serves an insurance function, defending the doctors who form its membership against lawsuits and malpractice actions, in return for a fixed annual fee. Its staff are a mixture of lawyers and doctors, led by lawyer Bill 'Happy' Henderson (Shane Bourne). He is ably aided by Dr Ella Davies (Kerry Amstrong), who later leaves to become head of emergency medicine at nearby St Albans hospital. Jason Donovan plays nemesis Richard Savage, a partner at Kato and Pittman, seen either as a wily and passionate advocate for patient rights, or an ambulance-chaser.