He meets with someone he had previously worked with to hunt down the kidnappers. After his partner turns on him, his partner and those working for the kidnappers are killed by a man in the woods.
Turns out the man is Castle’s father, who works as a spy. He works with Castle to get Alexis out of the house and kill the kidnappers. They won’t be able to see each other again but he is always there for him.
Castle and Alexis return home safe and sound with a cover story that does not include Rick’s father.