This game took place in the chess "Olympiad". This is the olympics of chess. The best players in each country play on a team against other teams. Each player plays his or her own games alone, but the team only wins if they win more games than their opponents. In this game, Pleci was representing his country of Argentina and his opponent, Lucius Endzelins, represented Latvia.
White has a temporary advantage because of his lead in development. But right now his bishop is under attack and so his knight. He can trade off the knight and move the bishop, but then Black will catch up in development and the game will be equal.
When you have a lead in development, and your opponent's king is in the firing line (like here) it is important that you use your imagination and try to find something. Even though White's pieces are "en prise", he also has more of them in the game, and more pieces can fight better! That's why Pleci now made a really surprising move.
White's rook broke into the Black position. But instead of capturing either sacrificed piece, Endzelins just attacked the rook. How many games have been lost because someone made a clever sacrifice but totally forgot that their opponent did not have to take it!? Suddenly, White has three pieces en prise! Before playing 11.Rd8 he had to have some plan to answer 11...Be7. Which he did! Check out Part 2 for the rest of the game.