The Law of Love mixes science fiction and magic realism with a sprinkling of New Age philosophy as it tells the story of Azucena, an astroanalyst in twenty-third-century Mexico City. Throughout the novel Azucena tries to help others as well as herself to remember and cope with their past lives. Along the way, she struggles to escape villains who threaten world peace and to reunite with her "twin soul'' Rodrigo. Her multimedia story, interspersed with color illustrations by Spanish artist Miguelano Prado and poetry and accompanied by a CD that contains arias by Puccini and Mexican danzones, ultimately asserts the unifying power of love.

The novel begins with the Spanish wresting control of Tenochtitlan, Mexico from the Aztecs in the sixteenth century and constructing a new city—which will eventually become Mexico City—upon its ruins. Rodrigo Diaz, one of Cortes's captains, demolishes a pyramid on a site where the Aztecs had conducted pagan ceremonies honoring a goddess of love, and builds his house there. As Rodrigo moves the stone that had formed the apex of the pyramid, he sees Citlali, one of his Aztec slaves, and rapes her on the spot. During the conquest of the city, Rodrigo had killed her son. Eventually, Citlali takes her revenge by killing Rodrigo's son. When his wife Isabel learns that after her son had died, her husband murdered Citlali and then took his own life, she does not have the strength to live.

The story then shifts to Mexico City in the twenty-third century and Azucena's pursuit of Rodrigo, her ‘‘twin soul.’’ Anacreonte, Azucena's Guardian Angel, explains the spiritual system of this future world, governed by a ‘‘Divine Will’’ or cosmic order that has been disrupted by "Wreckoncilers" who substitute ‘‘lies for truth, death for life, and hatred for love inside our hearts.'' The Wreckoncilers are allowed to ‘‘straighten out their screw-ups’’ through several reincarnations ‘‘until finally they learn how to love.’’ Anacreonte insists that Azucena and Rodrigo both still have ‘‘some outstanding debts to pay.'' The twin souls enjoy one blissful meeting with each other before Rodrigo disappears and Azucena begins a quest to be reunited with him. Isabel Gonzales, candidate for Planetary President, has exiled Rodrigo to the penal planet Korma, where he is unable to remember anything about his past. As she searches for Rodrigo, Azucena discovers someone is trying to kill her. With the help of Cuquita, her apartment building superintendent, she changes bodies in an effort to fool her pursuers. After Azucena spots Rodrigo on a newscast about Korma, she and Cuquita travel on an interplanetary spaceship to the planet. There she listens to music and regresses back to a past life during the Mexico City earthquake in 1985 that caused the death of her parents. She sees herself as a child who is killed by Isabel. Azucena then realizes that Isabel has hidden her violent past and probably has ordered her assassination. Rodrigo escapes the planet undetected after he exchanges bodies with Cuquita's husband. During the flight home, Azucena helps him regress into the past where he remembers that he was a pregnant woman raped by her brother-in-law, who was Citlali in a past life, and that he had raped her in the ruins of Tenochtitlan. Citlali and Rodrigo forgive each other for their past crimes.

After returning to the earth, police storm Azucena's building, Azucena and Cuquita's grandmother dies, and Azucena's soul takes her body. Teo, an undercover Guardian Angel, notices Azucena's acute jealousy over Rodrigo's attentions to Citlali and comforts her. Azucena regresses and discovers that she was Citlali's murdered child in a past life and that Rodrigo was the Conquistador who had killed her. During a televised debate between the two candidates for Planetary President, Azucena watches as Isabel's regressions become unblocked and projected on the screen. When Azucena then regresses, she realizes she was the daughter Isabel tried to have killed. She also discovers that Citlali had murdered Isabel's baby (Rodrigo's child), and Isabel had died hating her. In their next parallel lives, Isabel and she had been brothers. Citlali had raped her brother's wife (Rodrigo), and in return, Isabel had murdered her. Azucena decides that ‘‘then the Law of Love had come into play to balance the relationship between them, causing them to be born as mother and daughter, to see whether those ties could ease the hatred Citlali felt for Isabel. However, Isabel had never loved her daughter. After she met Rodrigo and fell in love with him, Citlali and Rodrigo ran away together. Isabel finds them dead in the rubble of an earthquake and kills their child, her grandchild, who turns out to be Azucena.

During her trial, Isabel is found guilty of her past crimes. When Azucena offers her forgiveness, Isabel allows love to enter her heart. Rodrigo's memory returns and he recognizes Azucena as his twin soul. After he finds and replaces the capstone to the Pyramid of Love, the lost city of Tenochtitlan reappears and merges with Mexico City. Azucena is allowed to return to her original body and the reinstatement of the Law of Love creates harmony for all.