Isshin is a man of action and has no need for long-winded explanations.
Urahara introduces himself and leads Isshin and Ryuu to his store. Urahara explains the process of hollowfication and states that he can save Masaki's life, but Masaki will never be the same again. Urahara has a vaccine to stop the victims from being destroyed by the hollowfication process, but a shinigami influence is needed to keep Masaki from hollowfying. Urahara has a special gigai for Isshin that will be tied to Masaki's soul, but while Isshin in this body he will not be able to use his shinigami powers or even see hollows, and it means that Isshin will have to desert his position in Soul Society. Isshin says yes before Urahara finishes the whole explanation and he gets Urahara to start the operation.
So this is the story of how Masaki ended up with Isshin. It's funny how the only way to save Masaki is for Isshin to live in the human world and bound to Masaki for the duration of Masaki's life. It was just meant to be lol. Now Ryuu has no chance with Masaki because Isshin always has to be with Masaki, but then again it's like Masaski was in love with Ryuu. There goes the great plan for the future of the Quincies. I guess the fact that Isshin and Masaki are bound together means Ichigo will the characteristics of hollows, quincies and shinigamis.