In the "Resisting the Tea Act" section of the Wikipedia article, it notes that the East India Company was allowed to import tea directly into the colonies without first passing it through wholesale markets in England. The tea was then sold at a price of 2/- per pound, undercutting the price of smuggled Dutch tea, then priced at 2/1 per pound. This is secondary to the fact that the tea still was taxed at -/3 per pound, meaning the price was really 1/9 plus thruppence tax, but this tax was included in the sale price and paid quietly by the consignment agents after it was sold.

Mercantile pressure in America tried to force all the consignment agents in America to resign and were partially successful, except in Massachusetts, where the colonial governor urged the agents to stand their ground. The Sons of Liberty, an organisation of such people, attempted to force the first shipment aboard the ship Dartmouth to return to England without unloading its cargo (and thus causing the tax to be paid).

Being unsuccessful, the Sons of Liberty snuck aboard the Dartmouth, which was anchored in Boston Harbour but had not unloaded its cargo yet, and destroyed the tea aboard.