Actor Daniel Faraldo signs the script, together with the director himself, Ramón Térmens. The illegal woman is a European co-production between Spain (Segarra Films) and the Czech Republic (Mímesis). The film has been shooting in the Modelo prison in Barcelona.

The life of Fernando Vila (Daniel Faraldo), an immigration lawyer, changes irremediably when he meets Juliet Okoro (Yolanda Sey). She is a Nigerian immigrant who works as a prostitute and asks for help to get her residence permit. While Fernando gets closer to his limit every day when he sees the injustices suffered by his clients, Juliet realizes that the mafia that controls her will never let her escape, since she is an indispensable piece in the web of corruption between the master of the brothel where she works and an unscrupulous police commissioner. Once they reach a point of no return, Fernando and Juliet decide to confront them and take the entire mafia to court, putting their own lives at stake.

The cast is completed by Klaudia Dudova as Zita Krasniqi, a Kosovar client of Fernando and Isak Férriz as Oriol Cadenas, the corrupt commissioner.