31 Dias (31 Days) is the story of Eva (Irán Castillo), a young and beautiful psychologist and author of a series of self-help books about modern women and their love relationships. For her next bestseller she has decided to prove her most daring theory: After the first 30 days together love ends, so the only way to have a meaningful relationship in this new millennium is when the couple part ways after the first month. At a magazine's photo session Eva meets Adam (Lorenzo Balducci), an attractive Italian photographer, and she decides he's perfect as her study subject. She will live a very intense relationship with Adam during 31 days, in order to prove her theory is right. However, what she is not aware of is that Adam is using her own advice from her books in order to make her fall in love with him and teach her a lesson. What neither of them suspects is that they'll come to learn that love has no rules.