The last episode of "Hawaii Five-0" delved into the death of Chin's father, who was murdered 15 years ago. 

The episode began with a bandit running from a convenience store as the store's owner called 911 to report the robbery, as well as the shooting of Lt. Kelly. The shooting happened 15 years ago, as it's revealed that the victim is Chin Ho Kelly's (Daniel Dae Kim) father, who later died from his gunshot wound.

The series then switches back to the present, in which an Internal Affairs investigation is ongoing into Chin Ho's activities regarding the arrest of the killer of his father, Malia's (Reiko Aylesworth) brother, Gabriel.

A new case had begun against Gabriel when the team found a tortured body. The body was identified as Amos Coy, a man who was involved in the Mexican drug cartel. It turns out Gabriel was his boss. 

The episode then goes back to a flashback in which Chin and John McGarrett are searching for Chin's father's killer. A young Kono tells Chin and John that a tattoo on one of Gabriel's associates is also worn by Gabriel, which is the mark of a local gang. 

Back in the present day, Chin is being interrogated, and tells the IA investigators that he did not have any communication with Gabriel after Gabriel left to work on the mainland, while he was married to Malia. 

The episode then shows when McGarrett (Alex O'Loughlin) visited the home of Amos Coy's wife and son. His son was being held hostage by two gunman, and his wife was taken to a local bank by Gabriel. Gabriel wanted Coy's wife to get $2 million from the bank that Coy had stored for him. He showed her a photo of Coy's tortured body, and said her son would suffer the same fate if she did not get him the money. 

But before she could, Five-0 arrived and chased Gabriel to a nearby roof, where they shot him in the leg. Gabriel then offered Chin $25 million if he would let him go. Chin refused and proceeded to handcuff Gabriel. 

The episode ended in the present day, with Chin in reverie about the day he met Malia. 

On the next episode, "Na Hala a Ka Makua (Sin of the Father)," "McGarrett and Danny are kidnapped by an escaped convict who wants Five-0 to prove he is innocent. Meanwhile, Grace gets into trouble when she punches someone at school."

TVLine also revealed that Episode 18 will see Chin and Jerry (Jorge Garcia) attending their high school reunion, where a murder will take place. The episode will also involve a flirty classmate who fancies Chin. 

Watch the promo for "Na Hala a Ka Makua" below, which airs Friday, Jan. 31 at 9 p.m. on CBS.