Maria Liz lives in Mexico City, in a very modest apartment with her grandfather, an English gentleman called Henry Alexander. When Maria Liz was only 12 years old, her father abandoned them and her mother fell into a deep depression and died shortly after. Mr. Alexander had recently lost all his money in a stock market crash, and the added blow of his daughter's death affected his mind. Through sheer effort and determination, Maria Liz managed to graduate from nursing school, but she clings to her dream of studying medicine. With her meager salary she supports her grandfather, who keeps a bag full of stock bonds and is sure that someday he will get his fortune back. Maria Liz also helps out her boyfriend Marcelo, who is studying to be an engineer. But Marcelo is a self-centered young man, and he only thinks of graduating and moving away as soon as he finds a good job, without regard to the sacrifices that his parents and Maria Liz have made in his behalf.

In a different part of the city, Sebastian lives with his mother, Doña Lucrecia, in a rich neighborhood. Sebastian is a civil engineer; an honest, hard-working man who is sure of himself and despises the false world that his mother lives in. After her in-laws robbed her of her late husband’s fortune, Lucrecia worked and struggled, and managed to keep up the appearance of a wealthy life.

Marcelo finds a job at the construction company where Sebastian works, and immediately forgets his promise to marry Maria Liz. He sets a plan into motion to steal away Sebastian’s position... and his fiancée, Araceli, the daughter of the owner. When a worker is injured in an accident, Sebastian takes him to the hospital where Maria Liz works. Although their first encounter is far from cordial, Sebastian realizes that she is an excellent nurse and soon hires her to look after his mother. But Doña Lucrecia begins to make the girl’s life impossible when she realizes that her son is falling in love with Maria Liz, and she returns that love. Doña Lucrecia wants her son to have the financial security and status that he would obtain by marrying the frivolous Araceli, but in Maria Liz, Sebastian has found the woman of his dreams, a kind-hearted, strong and sincere woman to share the treasure of "Priceless Love".