At the end of Season 20, Nick Torres learns that his mother's former boyfriend is up to his old tricks.  And as that season ended, Nick confronted him with a gun and threatened to kill him. 

The new season opens in Miami with a young Nick arguing with his mother.  In walks his mother's boyfriend and almost beats him.  Afterward, he takes out a gun, but his sister stops him and says that they will get justice one day.

Present day: After drinking in a bar, Nick is woken up at his desk by Parker, who is then called into Vance's office because the head of NCIS Cyber quit, possibly because Parker suggested that he could do better in the private sector. 

But without a head of Cyber, Vance taps McGee to fill in and tells Parker that he needs to find a new Cyber head. With McGee and Jess in the basement with the other tech trolls, the FBI enters the building and arrests Nick for murder. 

Although NCIS is officially barred from working the case, Vance tells them to do whatever they need, and he won't assign any new cases while they do.

Nick's sister Lucia, who is now a lawyer, comes to HQ and gives the team background on the man who abused their family, Maurice Riva.

He claimed to help them when they arrived in Miami with nothing, but then, when Nick and Lucia's mother's identity was stolen and she wanted to go to the police, Riva threatened to deport them. They vowed to get justice one day.

Without the FBI case files, the team doesn't have a lot to go on; the only evidence placing Nick at the scene is a fingerprint on Riva's ring, but nothing else. Although Jess and Jimmy manage to waylay Riva's body between the morgue and the funeral home so Jimmy can find out what he can. 

Despite the lack of evidence, Torres requests a closed hearing and pleads guilty, landing him back at Brunswick prison, the same place where he was undercover and where he learned about Riva's activities with Reymundo, Riva's new mark.

They bring in Marta DeLeon, Reymundo's mother, and she offers to bring Riva's laptop to NCIS. After some digging, the team learns that Lucia left Nick a message saying the same thing "algun dia." 

The team and Nick believe that Lucia killed Riva, which is why Nick pled guilty to protect her. When Nick gets stabbed in prison, Parker, McGee, and Lucia confront him and learn that he beat up Riva but left him alive. 

But Lucia didn't kill him either.  

After talking to Reymundo, Nick realizes where Riva has been keeping the money from his fraud schemes, and the tech trolls find the bank account in Belize, with the last access right around the same time as the murder. 

The team runs to Marta DeLeon's apartment, but she isn't there; the murder weapon is, along with a full confession written to the Torres family. Nick is cleared of all charges, and even when he thinks he's been fired, Parker welcomes him back.