Lord Vishnu took the Kurmavataram to provide the Devas with Amirtham, which will give them everlasting life and bliss. He also took the form of a beautiful Mohini in the course of distributing the amirtham between the Devas and the Asuras.   

The Devas and Asuras were constantly at war and the Devas soon began to lose their upper hand. Further, Indran was weakened by a curse by sage Durvasar.   

The Devas prayed to Lord Vishnu who bade them to churn the ocean (Paarkadal) and obtain amirtham, which alone could provide everlasting life and happiness. He further asked them to involve the asuras also in the task.

Devas were shocked by this, but Lord Vishnu promised them that he will make sure that only the Devas get to drink the amirtham. 

The Devas and Asuras got together to churn the ocean. They used Mandramalai to churn the ocean. Vaasuki, the snake agreed to form the rope for churning, in return for some amirtham. They started churning with Lord Vishnu and the Devas on one side (tail end) and the asuras on the other side (head end). But soon, the Mandramalai started sinking due to its great weight. Lord Vishnu, took the form of a tortoise (Kurmam-tortoise) to form the base and to hold up the Mandramalai, preventing it from sinking. As they continued to churn the ocean, Vaasuki the snake began to tire and started spewing its poison. The Asuras who were on the head side of the snake, suffered the most and they ran away in fear. The Devas prayed to Lord Siva for help. Siva came and consumed all the poison. Goddess Parvati rushed and held his neck so that the poison would not spread to the whole of his body. Thus Siva became blue up to his throat and is also known by the namavali "Neelakandan" (neela-blue colour from poison, kandam-throat). 

Once again the Devas and Asuras resumed the churning, when several valuables began to emerge. Kamadhenu (the cow), the provider of all wishes was taken by the rishis to aid in their yagas and pooja. Uchaisiravasu (white horse) was taken by the Asura king Bali. Iravadam (white elephant) was taken by the Deva king Indra. Kowsthubham (red jewel stone) was worn by Lord Vishnu on his chest. The Paarijaadha tree that grants all wishes went to the Deva Logam. Several Apsaras emerged. Devi Lakshmi then emerged in all her glory and beauty, stunning all. She garlanded Lord Vishnu, who placed her in his heart. Vaaruni Devi appeared next and she was taken by the Asuras. After this, Danvantri Bhagavan arose with a golden pot of Amirtham.  

The Asuras greedily grabbed the Amirtham pot and vanished. In order to retrieve the Amirtha kalasam, Lord Vishnu took the form of a dazzling beauty (Mohini Avataram). The Asuras falling for her beauty, entrusted her with the job of distributing the Amirtham. Mohini started serving the Devas first. The Asuras waited their turn in vain. However, one Asura called Rahu realised that they were being cheated. He changed his form and took a place between Surya (sun) and Chandran (moon). On learning that Rahu had consumed Amirtham, Lord Vishnu cut off his head with his Sudarshana Chakra. Since Rahu had consumed Amirtham, the head and the body continued to live. Brahma fitted the body of a snake to the head of Rahu and the head of the snake to the body of Rahu. Thus Rahu and Ketu, the two grahams were formed. The Devas with the power of the Amirtham drove the Asuras away to the Paadhaala Logam.