Jake's cousin, Bernadette, has to go to the hospital for a hip-replacement, leaving Jake, Mal, and Rose to care for her young'un, six year-old Mikey.
The too-precocious-for-real-life Mikey at first seems like something of a burden to Jake and Mal who aren't used to dealing with kids. Eventually, after he takes an interest in their work, they realize that they can use him to help them in their latest case.
Over the course of the case the three of them form an unbreakable bond of family and maleness that will last the rest of their lives.
Oh, and Mikey says many funny and cute things throughout the episode, mostly at Jake's expense.