I believe that the main conflict of the story is the inner turmoil in Buck - he was domesticated to an extent, but he still feels the urge to respond to "the call of the wild" and join his fellow wild dogs. The plot revolves around his indecision to choose where his life will be heading. He is torn between his master and his primal instincts to run away and be wild. The conflict is resolved when he finally accepts what he is and joins the wilderness - however, he hasn't completely forgotten his former master, as he returns to Thorton's grave every year.
