There are only two original accounts of the first Thanksgiving.

Edward Winslow, writing in "Mourt's Relation".

William Bradshaw, writing in "Of Plimouth Plantation".

Bradford does not even mention the celebration meal, though he does talk about the good harvest after the previous hardships:

""They begane now to gather in ye small harvest they had, and to fitte up their houses and dwellings against winter, being all well recovered in health & strenght, and had all things in good plenty; For as some were thus imployed in affairs abroad, others were excersised in fishing, aboute codd, & bass, & other fish, of which yey tooke good store, of which every family had their portion. All ye somer ther was no want. And now begane to come in store of foule, as winter approached, of which this place did abound when they came first (but afterward decreased by degrees). And besids water foule, ther was great store of wild Turkies, of which they tooke many, besids venison, &c. Besids, they had about a peck a meale a weeke to a person, or now since harvest, Indean corn to yt proportion. Which made many afterwards write so largly of their plenty hear to their freinds in England, which were not fained, but true reports."

Winslow gives the only account of the feast with the Indians:

"our harvest being gotten in, our governour sent foure men on fowling, that so we might after a speciall manner rejoyce together, after we had gathered the fruits of our labours ; they foure in one day killed as much fowle, as with a little helpe beside, served the Company almost a weeke, at which time amongst other Recreations, we exercised our Armes, many of the Indians coming amongst us, and amongst the rest their greatest king Massasoyt, with some ninetie men, whom for three dayes we entertained and feasted, and they went out and killed five Deere, which they brought to the Plantation and bestowed on our Governour, and upon the Captaine and others. And although it be not always so plentifull, as it was at this time with us, yet by the goodness of God, we are so farre from want, that we often wish you partakers of our plentie."

http://www.pilgrimhallmuseum.org/pdf/TG_What_Happened_in_1621.pdf

Those are the only eyewitness accounts we have, both are brief, and Winslow's is the only one which mentions the feast.

Of course, we know some more about the context. For example, present at the First Thanksgiving were 53 Pilgrims, plus the 90 Indians mentioned by Winslow. So, there were more Indians than Pilgrims present.

The 53 Pilgrims were the only survivors of the 102 colonists who had landed about a year earlier.

http://www.history.com/topics/mayflower

No other ship had come to Plymouth with relief or additional settlers before the First Thanksgiving.

The Indians, also, were the shattered remainder of a much more populous people. A plague had hit the Massachusetts coast in 1617, and devestated the people living there, so the Pilgrims landed in depopulated lands.

So, that is all we know about the "true" story of the original Thanksgiving, and it is not very much. Of course, many traditions and stories have clustered round the event some of which might be correct, and some just legends.