The Departure of Boromir is the first chapter of Book One of The Two Towers.
Aragorn finds Boromir wounded with many arrows, who tells him that Orcs took Merry and Pippin, and they were still alive. Boromir dies, and his body is set down the stream on a 'funeral boat.'; Aragorn, Legolas and Gimli decide to follow the orcs who had captured Merry and Pippin, rather than following Frodo and Sam. The three of them set off to chase the orcs.