Devaki had a brother, Kansa, a tyrant, who along with some other demon kings was terrorising Mother Earth.
Kansa had usurped the throne of Mathura from his father, the benevolent King Ugrasen. Mother Earth took the form of a cow and went to Lord Brahma, the creator God of Hinduism, with her plight.
Lord Brahma then summoned Lord Vishnu, who assured Mother Earth that he would take birth as Lord Krishna to end this tyranny.
Kansa agreed to let Devaki get married to Yadava prince Vasudeva in the hopes of taking over the Yadava clan too. When Devaki was getting married, Kansa was told by fortune tellers that one of the offsprings of Devaki would bring his end.
In his paranoia, Kansa brandished his sword and decided to kill Devaki right there and then. But after Vasudeva begged for his wife's life and promised to hand each child to Kansa as soon as it was born, Kansa let go of his sister, and instead imprisoned Devaki and Vasudeva, and made sure that none of Devaki's children survived.
As soon as a child was born to Devaki, Kansa would smash the child's head against the walls of the prison.
However, on the night of Krishna's birth, as soon as Krishna was born, a bright light filled the prison and Vasudeva was woken up by a divine voice that guided him to take Krishna across the Yamuna and leave him with his dear friend Nandraja, the head of the Gopa tribe.
Nandraja and his wife Yashoda had also given birth to a baby girl that night, so Vasudeva secretly carried baby Krishna across the Yamuna river, which was no longer in its calm state, but instead was raging as if it were the ocean.
Just then Lord Vishnu's Shesh Naag, the giant multi-headed snake came and helped Vasudeva carry Krishna safely across the river. Vasudeva went to Nandraja's house and exchanged the babies.
His heart was filled with a deep sadness, as if he had left a part of his soul behind. He headed back to the prison with the exchanged baby, who gave a loud cry as soon as she lay next to Devaki. The guards informed Kansa that Devaki's eight child was finally born.
Devaki begged Kansa not to kill the baby, she pleaded that the prophecy must have been wrong as her son was meant to bring the end of Kansa but to no avail.
When Kansa tried to kill the exchanged baby, it transformed into Goddess Durga and warned him that his death had arrived in his kingdom, and would come back to punish Kansa for his sins.
Knowing that Kansa would find no peace and keep thinking about his end, she said that she could kill him then and there, but Kansa's end had to be timely, and then the goddess vanished.
However, Kansa was certain that the prophecy may not be entirely true given that if his slayer was born inside the prison, he would have surely killed him.
With a slight relief, Kansa finally freed Vasudeva and Devaki, and let them live in a separate palace. After some days, Vasudeva shared what happened on the night of Krishna's birth, and though Devaki was sad, she was relieved to hear her son was safe.
Days later, news of Nandraja and Yashoda's son's birth reached the kingdom, people were gushing about the unique twinkle in the child's eyes, about how he was always happy and how his mere presence spread joy and cheer all around.
Krishna grew up far away from all the chaos of Mathura, in Gokul, with Nanda Baba and his wife Yashoda as his foster parents.