After an opening action scene in which Catherine (Michelle Borth) goes undercover for Five-0 to get information about Sato and has to fight for her life in the men’s room of an underground gambling club with a less than forthcoming member of the Yakuza (Brian Tee), most of “Un Nalohia” deals with the unexpected arrival of Steve’s sister Mary (Taryn Manning) and her new baby which McGarrett (Alex O’Loughlin), Chin Ho (Daniel Dae Kim), and Danny (Scott Caan) are forced to look after while investigating the murder of a surfer (and current undercover agent for the ATF).
Danny and McGarrett talk with the victim’s wife (Floriana Lima), partner (Christie Burson), and handler (Colby French) on the gun-smuggling ring case Chin-Ho’s old friend was working on when he died. They also discover $100,000 in cash and an expensive condo rented out from a local real estate agent (Daryl Hannah) for only a single day the make them suspect someone in the operation is dirty. After discovering their victim made the same conclusion before his death setting a trap to catch them in the act, McGarrett and his team sit back and grab the perpetrator who, as they suspected, was indeed an inside man.
The Three Men and a Baby antics are rather cheesy, and McGarrett’s continued attempts to pawn off his duties as babysitter to other members of the team don’t cast him that good of a light (although his obvious concern for Joan’s well-being helps slightly mitigate it). Catherine’s butt-kicking and information gathering gets her an extended stay with Five-0 as McGarrett offers his girlfriend a full-time position (also helping to fill in the gap until Grace Park‘s return). The episode also continues the dysfunctional relationship between McGarrett and Captain Grover (Chi McBride) who raises concerns once again with Five-0’s dangerous tactics and suggests to Chin-Ho that one of these days back-up may not arrive in time if McGarrett doesn’t learn to play by the book.