When Voyagers real doctor is killed its time for some emergency plans. Luckily sick bay is fitted with an EMH (Emergency Medical Hologram) for these situations. And with a some major reprogramming The Doctor's programme manages to keep running constantly.
The doctor soon developers a personality and tries to cope with normal Star Fleet life while the notable problem of being a hologram. A mobile emitter is developed to allow him to go beyond the confines of sick bay.