Anthony DiNozzo (Michael Weatherly) bids farewell Tuesday, May 17, (CBS/2 at 8 p.m.) to the NCIS team, as the manhunt for an escaped British spy reaches a climax in the Season 13 closer. DiNozzo’s father, Anthony DiNozzo Sr. (Robert Wagner), is back for the farewell. (Weatherly will star next season in a new CBS drama, “Bull,” about Dr. Phil McGraw’s early career as a trial consultant.)

Anthony DiNozzo was — and has been these past 13 seasons — everyone’s brother and son. To fans, he was also the frat boy, the cut-up, the guy in the office (possibly your office) who didn’t just spend a lot of time at the water cooler, but all his time at the water cooler — testing his one-liners, or maybe just testing your patience. Nevertheless, after Leroy Jethro Gibbs (Mark Harmon), DiNozzo was also the most important character on CBS’ most important drama. He was that critical to its success, and the reason why his departure is a big deal.

Characters come and go on TV series. “NCIS” will go on, too. That’s a given. But absent DiNozzo — and Weatherly — it will absolutely be a different series. That’s also a given. Like all enduring comic relief characters, DiNozzo was the entry point, or the welcome mat, for a lot of fans. He was the one who effectively told them that it was safe to come inside, no matter how much catastrophe, death, mayhem, violence or destruction was sandwiched in between those solidly middle-class commercials for lawn mowers or cars.

At first, the audience, or female audience, hated him. In the early days, DiNozzo was a cad, a commitment-phobe who joked about his various conquests. The senior field agent of the Major Case Response Team then slowly began to grow up. He had to. His complicated relationship with Ziva David (Cote de Pablo, who left a few seasons ago) softened his edges. The relationship with Gibbs added the facets.

In an early — and famous — episode, “SWAK,” DiNozzo contracted a deadly plague, and Gibbs’ frantic effort to save him seemed almost like the efforts of a father about to lose a cherished son. DiNozzo often returned the favor. When he dived into the cold depths to save Gibbs, who was trapped in a car (“Requiem,” Season 5), he wasn’t just saving his boss. He was saving Dad.

Say goodbye to that father/son dynamic on Tuesday. Jethro — a somewhat lonely, ascetic soul even in the happiest of times — will have to say goodbye, too.

Where will Tony go? Maybe back to Long Island. (That’s right. He’s from LI.) But you can be certain he will come back to “NCIS” for a cameo.

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Dear old Dad would insist.