This poem is expressed in a conversation between Fergus and a druid. The druid, at first only asks him what he would like to do.
Fergus says that he has followed the druid for the whole day as he changed shapes, and that now he finally holds a human form. He recounts how young Conchubar sat at his side, and seemed so wise that he gave his crown to him, to ease his own sorrows. He tried to become one of the people, but failed, still feeling like a king. Fergus then expresses a desire to be as wise as a druid, despite the druid's warnings that such wisdom severs one from humanity.
The druid gives him a bag of dreams to open. Fergus sees what he has been in his life, but sees it all as a web of sorrow. Knowing all, he is filled with sadness.
This poem primarily treats the isolation of a king who is weary of his rule and his social role. King Fergus is an Irish historical figure who figures in the Tain. Fergus fell in love with Ness, and gave up his throne to Conchubar, who was the son of Ness by another marriage. Myths look on this variously as an usurption and as a source of great happiness for Fergus, who did not enjoy being king.
Yeats's version of the myth is somewhat consistent with both interpretations. Fergus is ambivalent about whether he did the right thing in surrendering his throne. He has not assimilated into non-royal society. He seeks the help of a druid, an ancient healing or religious figure in Celtic societies, to clarify whether he has made the right choice.
The druid's help, which comes in the form of a "slate-colored thing" which refers both to the bag of dreams and, perhaps, to the grave. With the help of this bag of dreams, Fergus "know[s] all" at the poem's end. But this knowledge does not quell his anxiety; rather, it sinks him into depression. By knowing all, he has robbed himself of the hope that comes with uncertainty. He is as sure as death, and as futureless.