27 Bhagavatha -- Matsya Avatara

King Parikshit, asks sage Shukha: "O divine sage, I now wish to hear from you the story of the very first avatar of the Lord as the mighty fish. Why is it that he preferred  the fish which is tamasika (quality of dullness, ignorance, inertia) by nature?"
Sage Shukha continues his narration of the Bhagavatha, the stories of the Lord.
The almighty assumes various forms to uphold dharma.  At the end of Brahma's day (which lasts a seemingly countless number of years), when he retires for the night, there comes a time when the entire earth is washed away for regeneration, devoid of any activity and the absence of the three gunas.
Brahma, tired after his long day, unconsciously murmurs the vedas. The demon Hayagriva (with a horse's head) steals the vedas (knowledge) by dint of his yogic power. The Lord Srihari assumes the form of Hayagriva and vanquishes the demon Hayagriva. Then he assumes the form of a fish and retrieves the vedas from the dreadful deluge.  He continued to be in the waters until the next creation.
In the kalpa (a measure of time) which just concluded, there was a royal sage Satyavrata. He was devoted to Lord Narayana.  One day, when he was performing austerities in the river Kritamala (a river in the south of India), a fish got into his palms while he was offering water to his ancestors. As he prepared to drop it back into the water, the fish spoke: "I have sought refuge in you, O King. Please don't throw me away at the mercy of those aquatic creatures who kill their own species." The merciful king put the fish into the kamandalu and carried it to his hermitage. 
The fish grew bigger and again appealed to the king for bigger space. The king shifted the fish to a pitcher. In less than an hour's time, the fish grew to three cubits in length, necessitating a further change in accommodation. The fish was shifted to a pond, and then a lake. The fish kept growing to monstrous levels occupying fully the space it was provided. The time came when it had to be shifted to the ocean. The fish said, "Do not leave me here, O valiant King! The mighty alligators and other aquatic animals pose a severe threat to my life."
Satyavrata asks the fish, who had by now become a huge whale with a golden hue: "Your exploits as the great fish makes me conclude that you are Lord Narayana himself. Hail to you, O supreme Person! Your descents on earth is always for our benefit. I wish to know the purpose for your avatara as the Matsya."
The Lord replied: "On the seventh day from now, the terrestrial regions will be submerged in the waters of the deluge. I will be sending you a spacious boat which will approach you. Collect all the herbs, the annual plants, seeds of all types, the seven seers, animals of all types and board that vessel.  Sail undaunted in that expanse of water, which will be devoid of light. You will be guided solely by the effulgence of the sages aboard.  Attach the vessel (which will be tossed about by the gales), by means of the serpent Vasuki, to my horn. I shall be present in the water pulling you all, as long as the night of Brahma lasts. In the course of the journey you will realise the transcendent reality -- the para Brahma." And the Lord disappears.
Satyavrata awaits the time for the journey contemplating on the divine Matsya.  Clouds gathered and the rains poured. The ocean began to rise steadily inundating the earth on all sides. Satyavrata did as he was commanded and boarded the boat along with the sages, herbs, animals etc., The sages prompt him to meditate on the Lord to help overcome the calamity.  Concentrating his mind deeply on the Lord, Satyavrata finds the Lord again, as the dazzling golden whale.  He fastens the boat to the horn of the divine whale (with Vasuki as the rope).  He prays to the Lord to help him cut the knots of ignorance and seek the ultimate truth.
The Lord teaches the highest truth to Satyavrata, and reveals the mystery of his own self, elaborated in the Matsya Purana.
When Brahma arises from his slumber at the end of the deluge, Lord Hari restores the vedas to him.
Satyavrata, in the present Kalpa, is Vaivaswatha Manu (the son of Surya, the sun) or Shraddhadeva.
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What the allegorical story means:
    1. The fish represents the different propensities of the mind, turbulent until it becomes spiritual, guided by the guru.  Otherwise it keeps on outgrowing whatever other space it is provided with.
    2. Bhagawan Matsya is the sadguru.
    3. Satyavrata is the seeker(the soul, jeeva) in quest of Truth.
    4. The boat is the body and Vasuki is the mind itself.
    5. The ocean is the tumultuous passage of life.
    6. The saptarishis (seven sages) are the symbol of ethics and spiritnual endeavour.
    7. The sleep of Brahma means that a person overcome by inertia loses spiritual knowledge. Vishnu restores that knowledge.