McGarrett's sister Mary returns for a visit. But he's annoyed that she's staying at an Airbnb instead of his house. A man carjacks a ride-share vehicle and its driver. Both the driver and the carjacker are found shot dead. The carjacker, Carter Hill, was a visiting statistical analyst. Mary is suspicous of her next-drive neighbor, Ben Miller, who argued with her Airbnb host until  the host wants to kick out Mary. So she and Steve confront Miller. He agrees to let her stay, but now McGarrett is also suspicious about him. Noelani finds cocaine and PCP in Hill's system. Tani and Junior discover female blood in Hill's motel room. Mary and Steve meet a boy who threw a rock with a note on it at them. He wants them to get his drone back from Miller before he gives them the dirt on Miller he has. His intel is sketchy. Kira, the woman with whom Hill left the bar where he got the drugs, has a handgun and has disappeared. The team's theory is that she was alive but injured. She was stashed in the trunk and surprised and killed both the other two men. Eddie tracks down Kira, who is hanging from a branch on the side of a cliff. Mary breaks into Miller's basement. She thinks Miller has a meth lab. McGarrett finds a false wall which leads to that lab. Then Miller comes home, figures out someone has broken in and he and his wife grab guns. Mary creates a gas off the internet and they overcome Miller and his wife. Junior climbs down to secure Kira. Junior swings by to pick up Tani for the wedding. Mary decided to stay with Steve. Tani and Junior try to outdo each other dancing at the reception. They slow dance and almost kiss.