The title of this novel truly hints at its subject matter: a young man who is trying to make sense of his dad's lies, and realizes that all lies have their basis in reality.
In regards to plot, William Bloom is visiting Edward Bloom, the father, before he dies. William is trying to reconcile a life of being lied to by his father. Using many allusions to the Odyssey, Ulysses, and Hercules, William Bloom is finally able to decipher the true meaning behind all of his dad's stories. In fact, William tells the story in the first person. The reason why he is doing this is so that he can retell the stories of his dad by making them include the most truth possible. That is how William recovers from his life of being told many lies. What William ends up learning is that all half-truths have some root in the real world. What is important is to find that truth in the real world and explain it.
In conclusion, we should say that William only shows up in the story when he is talking about these vignettes that he calls "death takes." During these episodes, William is figuring out how this conversation will go with his father, and if he will be able to get to the root of the whole truth.