The legacy of Leroy Jethro Gibbs, originally played by Mark Harmon in “NCIS” and now portrayed by Austin Stowell in the prequel series “NCIS: Origins,” continues with a crossover event that sees Harmon reprise his role. Both shows tackle the same case in different time periods — it was first opened in the ’90s and it gets reopened in the present day timeline when Tommy “Peaches” Mulligan breaks out of prison despite having just three weeks left of his sentence. The “NCIS: Origins” Season 2 episode “Funny How Time Slips Away” and the “NCIS” Season 23 episode “Now and Then” both revolve around this case, which bedeviled Gibbs and fellow agent Mike Franks at the time. The gauntlet is picked up by the current “NCIS” team, who vow to bring in the escaped convict and figure out if he’s truly guilty. In the end, only someone who was there back in the day — Vera Strickland (Roma Maffia, reprising the role after 12 years) — can guide the team in the right direction.

It’s a story that ties heavily into several previous “NCIS” installments. Here are three episodes you should watch before seeing the “NCIS” and “NCIS: Origins” crossover.

NCIS Season 6, Episode 24 (Semper Fidelis)

You need to go back and watch “Semper Fidelis” because it introduces viewers to one of Gibbs’ many rules in an “NCIS” — Rule #11, “When the job is done, walk away.” He and Tony DiNozzo (Michael Weatherly) are talking after a difficult case when Gibbs tells Tony that this is how he manages to get through each trial and every single difficult day. Tony repeats that motto to himself during Season 7’s “Obsession” after having a similarly tough time. We actually see the formation of this rule in “Funny How Time Slips Away,” the “NCIS: Origins” half of the ….

The younger Franks (played by Kyle Schmid, who wears brown contacts to make him look more like original actor Muse Watson) tries to convince Gibbs to let go of his collection of pictures of victims, which he keeps near his tool bench and uses to motivate himself, even though they haunt him. Gibbs does so eventually — without knowing that Franks can’t let go of the case they’re investigating. He visits Tommy Mulligan once a year for decades, even as his temples grey and he becomes an old man, trying to figure out why he gave a false confession.

NCIS Season 8, Episode 23 (Swan Song)

Mike Franks is killed off in “Swan Song,” and it focuses heavily on who he is and what he means to Gibbs. This makes it a definite must-watch for anyone who needs to understand how much Franks influenced Gibbs and why his death is so devastating to the “NCIS” canvas at large. During the episode, the crew are forced to deal with the Port-to-Port Killer, Jonas Cobb (Kerr Smith), who recently murdered a minor member of the NCIS team to gain access to the interior of their offices.

The investigation leads the team to realize one of their own might be in the crosshairs of the serial murderer, who is looking to specifically claim revenge on Gibbs and will do anything he can to reach that goal. Franks is the target, but he goes down fighting, and he leaves behind a clue for Gibbs to latch on to in order to bring his murderer to justice. If you’ve ever been curious about Franks, this is a go-to episode, and it’s a must-watch ahead of the crossover.

NCIS Season 11, Episode 3 (Under the Radar)

Roma Maffia’s Vera Strickland becomes the team’s go-to source while they try to solve the mystery behind Tommy Mulligan’s escape from prison. She ends up being quite the helper and her ability to call things as she sees them keeps the whole team on their toes. If you want to see Vera in action before the crossover — and, technically, before her first appearance in “Origins” — she shows up in “Under the Radar,” a Season 11 episode of “NCIS.”

Vera has an entire subplot in “Under the Radar” in which she’s forced to work at the office before she can claim her retirement benefits. To say she’s grumpy about it all is putting it mildly. While investigating a case, she ends up getting injured — and she blames Tony for her ills. Though it takes a little bit of convincing, she eventually comes around to Tony’s charm — though, as with all things Vera, she does so begrudgingly.