

Throughout the film Foster speaks only in Nell's unearthly-sounding private language, but is able to use this, together with gestures and facial expressions, to convey a full range of emotions. The most outstanding feature of the film is Jodie Foster's performance in the title role- a remarkable one even by the standards of this talented actress. In order to do so, they find that they need to learn how to communicate with her. The question they have to resolve is whether Nell should be committed to a mental institution or allowed to continue her life in the woods. They learn that she had a twin sister, May, who died as a child, and that her seemingly-strange speech is actually a mixture of distorted English learnt from her mother (who had a speech defect as the result of a series of strokes) and words remembered from a private language spoken with her twin. Jerry and Paula move into the woods to observe, and gradually start to find out more about Nell's past. The judge responsible for deciding Nell's future, decides that she should be kept under observation for three months so that more information can be obtained. A psychiatrist, Paula Olsen, sent to investigate the case, decides that Nell is mentally retarded, but Lovell, who is becoming increasingly fascinated by this strange young woman, contests this diagnosis.

Although in her late twenties, she has never been outside her home and the forests which surround it, and knows nothing of the outside world.

Nell is unable to speak English and can only speak an unknown language. The local doctor, Jerry Lovell, visits the property to certify the death and discovers that, unknown to the community, Violet had a daughter, Nell, living with her. An eccentric and reclusive old woman named Violet Kellty is found dead in her home, a wooden cabin without electricity or running water in a remote mountain are of North Carolina. 'Nell' tells the story of an even more extreme example of the same phenomenon. This was an example of an uncommon, but not unprecedented, phenomenon known as 'idioglossia' or 'cryptophasia'.

Even after they had learnt English, they continued to communicate with each other in their secret language, which was partly a mixture of distorted English and the German that was their grandmother's native tongue, but which also contained some inexplicable vocabulary items. The girls, identical twin sisters, had developed a private language quite incomprehensible to outsiders. In the late 1970s two young girls named Virginia and Grace Kennedy caused great interest in the academic disciplines of psychology and linguistics.
