Leo Billups, formerly known as Lonny McRoy, had the bad fortune of witnessing a drug-related murder that involved the Cruz family, one of the most powerful drug families in the country. It's a common enough story in WITSEC but what sets this one apart is that the person he saw murdered was his own mother... and Leo is eight years old.
When Leo enters WITSEC, no one's really sure what to do with him. His father can't go into the program, since he's one of the people Leo is testifying against. So of course, Stan McQueen, Mary and Marshall's boss, looks at Mary for the answer. Mary Shannon is not exactly the maternal type, although her caretaking instinct is a lot stronger than she would admit, probably because she doesn't even realize how good she is at taking care of everyone. Mary's bright idea is to ask (or rather strongly persuade) another WITSEC family if they'll adopt him. When they agree, it seems like everything's going to work out perfectly.
But Leo's father, Vernon McRoy, sues for custody, claiming the Federal government had no right to take Leo in the first place. Everyone knows he just wants Leo so he can keep him from testifying, but the law is on his side. Now, Mary must find a way to keep Leo away from his father and safe from the Cruz family, which seems like it's going to entail Mary playing yet another role: Sarah White, Department of Justice, Family Relations. Can Mary really pull off a "Sarah"? She'll have to if she wants to score a happy ending for this little boy.