'NCIS' star Sean Murray unpacks McGee's grand daddy of a twist (Spoilers)

Warning: Spoilers ahead for CBS "NCIS" episode "In Too Deep," now streaming on Paramount+

The March 17 "NCIS" episode"In Too Deep" dove into the personal life of Special Agent Timothy McGee (Sean Murray).

The surprising episode (now streaming on Paramount+) involves a previously unknown past love of McGee, the longest-running "NCIS" character. But the unfolding story also reveals a grown child from McGee's past that even the proud family man didn't know about.

"McGee, for 22 years of this show, has had this idealized version of the way his life is supposed to go, and things are compartmentalized," Murray tells USA TODAY. "He's married, he has kids already. So this episode throws him for a complete and total loop."

When did 'NCIS' McGee date Olivia Garcia ?

McGee is immediately struck when he finds the picture of Olivia Garcia on a body during a case. "She's an old girlfriend," McGee says of the never-before-mentioned past love. "We used to date back in the day, before I met Delilah."

McGee had a romantic "NCIS" fling with Abby Sciuto (Pauley Perrette) and married his pregnant longtime girlfriend, Delilah Fielding (Margo Harshman), in Season 14. The stable couple have two children, twins John and Morgan.

"This relationship must have happened sometime between Abby and Delilah if we're going by show history," Murray says. "And he dated Olivia for about six months."

At the time of the amicable breakup between Garcia and McGee, there was no mention of a child. Olivia found out she was pregnant after the split and never contacted her ex with the news.

What happened to Olivia Garcia in 'NCIS'?

Turns out she is missing, likely kidnapped. Or worse. The Trask Underground Research Facility (TURF) scientist has been missing from her biofuel research for weeks. McGee meets one of a clumsy TURF intern Pete, who is revealed to be Olivia's 18-year-old son, Matteo (Patrick Keleher), under interrogation.

"You can see McGee doing the math and putting two and two together," Murray says.

Stunned McGee has to leave the interrogation room; he's so shocked that he's likely been an accidental absentee father. But the NCIS crew perform a DNA test just to check.

Even before the positive results are in, Matteo turns out to be a chip off the old block by joining McGee in apprehending the kidnapper, a vengeful TURF staff cook. Matteo gets taken hostage but still helps his new dad take out the baddie. Showing slick moves and wielding a fire extinguisher, tech specialist McGee has been jumping into the fray over the past seasons.

"We need to see McGee involved in more of this kind of action because it's fun, and I do it pretty decent," Murray says.

Will son Matteo appear in future 'NCIS' episodes?

Olivia turns out to be kidnapped and alive. The reunited exes agree that McGee will have a role in his son Matteo's life going forward.

"McGee has already prided himself in being a good father, because his relationship with his own father was not wonderful," Murray says. "The idea that he has an 18-year-old son freaks him out at first, but he falls into the father role naturally."

We can expect Matteo to appear in further "NCIS" episodes, because the family news will continue to ripple.

"So much remains to be seen, such as: How is Delilah going to react to this?" Murray says. "We're not going to set this and forget it. We'll revisit this for sure."