This is an absolutely fascinating book.
Walker starts with a question: What makes great sports teams great? He came up with a criteria and looked at the history of athletic teams--national and international, men's and women's, all varieties, so long as they were a cooperative venture--and identified the most dominant dynasties of their eras. He found 122 teams that met the basic criteria, then identified 16 that stood out as the best of the best. He dubbed the 16 as Tier One and the remaining 106 as Tier Two.
Then he looked at the 16 teams to see if he could identify anything they had in common as a shared secret of their success. He noticed that the span of success for one team coincided with the membership of a particular player. Then he looked at the others. [They] weren't the only team whose Tier One performance corresponded in some way to the arrival and departure of one particular player. In fact, they all did. And with an eerie regularity that person was, or would eventually become, the captain. The more he looked, the more he found similarities between all of those figures, until he eventually had to conclude that the most crucial ingredient in a team that achieves and sustains historic greatness is the character of the player who leads it.