This happens about a third of the way into the poem. In line 1278, it is described how Grendel's mother, having dwelled on the fact of his death for some time, determines that she must get revenge for her son. Grendel's mother then comes to Heorot, seizes a shield-warrior, and takes him away back to her fen. Hrothgar then becomes afraid that the terror has returned to his people, and Beowulf is alerted to what is going on.
Beowulf promises Hrothgar that he will not allow Grendel's mother to terrorize Heorot as her son had done. He vows to follow and kill her, wherever she may hide. In line 1408, Beowulf's journey to her fen begins ("Oferéode þá æþelinga bearn," or "then the nobleman's child began to travel...").
In line 1473, Beowulf begins to give a speech about his intentions to kill Grendel's mother. he tells those who are watching what to do with his treasure if he should die in the effort.
Beowulf then spends "hwil daeges," the better part of a day, swimming towards the bottom of the mere. His battle with Grendel's mother continues for a significant section of the poem. It is in line 1593 that the waiting Danes see the water "brim blode," or colored with blood. They discuss this omen amongst themselves and decide that Beowulf will not return; they think "þæt hine séo brimwylf ábreoten hæfde" (that the sea-wolf has destroyed him—line 1599). Therefore, in the following lines, 1600–1601, it is described how the Scyldings abandoned the cape and set out for home.