Start your tour of the Tokyo Central Wholesale Market (colloquially "Tsukiji Fish Market") on at 0530 (Monday through Saturday) during the tuna auction in the inner market. Huge frozen fish carcasses are sold by lively auctioneers and their clipboard-toting assistants.
Once sold, the fish are marked with the name of the buyer.
At some point, the heads get either chopped off with a huge axe or sawed off with a circular saw.
Once the whole fish are sold to wholesalers, they go back to their stalls within the market to divide up the carcass into filets to sell to sushi markets.
The stalls within the market also sell every other kind of fresh--or live--seafood:
You can buy handmade knives in the outer market, or, if you've developed an appetite for raw fish, grab some sushi in one of the many little restaurants:
Getting there: Don't go on Sunday when the market is closed. Take a taxi if you want to arrive really early. Otherwise, the Tsukiji station on the Hibiya subway line will get you within a 5-minute walk.