Juan Carlos and Diego, now in their 30s, have been best friends since high school. Politically and ideologically, they do not always see eye-to-eye, but their friendship is sacrosanct. Juan Carlos is planning to marry his girlfriend, Monica. Since she entered his life, he, Monica and his friend Diego had a tradition to celebrate New Year’s Eve at Monica's family house on Lake Atitlán, a picturesque lake surrounded by mountains and nature. However, the engagement of Juan Carlos and Mónica has not prevented a secret mutual attraction from developing between Diego and his best friend’s bride to be. Using a narrative that progresses in reverse chronological order, and filming in only one location – Monica’s house on the lake – Fidelidad tells the story of a love triangle that pushes each character to the limits of their friendships, forcing each of them to question the meaning of love, friendship, morality, and the nature of fidelity.
The idea of betrayal is often linked to lust and sexual desire. Acting on our desire can cause jealousy, loss, all in the name of experiencing fleeting pleasure. While breaking promises made to romantic partners, explicitly or tacitly, is often painful, the price of betraying the bond of trust formed in a friendship can be just as serious. The characters in this film love one another, and while the betrayal that occurs between two of them is sexual, their relationships and friendships have failed on a deeper level, they lack ability to tell the truth, as both friends and lovers must do. Fidelidad challenges us to think about moments we haven’t lived up to the honesty that true friendship requires, and how all dishonest actions accumulate over time to cause great pain for those who we love.