My life for an infidelity

Marta is an attractive, forty-nine-year-old woman and the boss of a modest but profitable advertising company. She has a son named Julián and a daughter named Melisa, whom she adores.
But at a meeting with other businesspeople, Marta befriends Manuel, a man her own age, to such an extent that they agree to merge their companies by getting married.
Even so, one afternoon like any other, she meets Mario, to whom she is attracted, a man far younger than her. The attraction is mutual, but it remains nothing more than a fleeting passion, as they never see each other again.
On the day of Marta and Manuel's civil wedding, after the signing, the businessman's son, who was to be a witness, arrives late, and it is none other than Mario.
Marta signed a clause in the marriage contract stipulating that