In the beginning Bellisario created the ‘verse. Now the ‘verse was formless and empty, darkness was over the surface of the deep, and the Bellisario was hovering over the waters.
And Bellisario said, “Let there be JAG,” and there was JAG.
And in time JAG begat NCIS, and NCIS begat NCIS: Los Angeles and NCIS: New Orleans. After the initial NCIS Backdoor Pilot on JAG, JAG characters started appearing on NCIS. Similarly, following their respective pilots, NCIS:LA and NCIS:NO shared characters with their parent series, but oddly not each other. Also, a JAG character started cropping up on NCIS:LA. So thus we have (Note: the arrows show the flow of characters):
But, we also have JAG importing a character from the not-very-popular First Monday (I’m possibly the only person in the UK to have bothered watching it): Edward Sheffield (Dean Stockwell), later Secretary of the Navy.
And then the walls of the ‘verse were broken, when Hetty from NCIS:LA appeared on Scorpion, and a crossover happened between NCIS:LA and Hawaii Five-0 (the reboot of Hawaii Five-O).
Hawaii Five-0 opens up another can of worms though. Characters appeared in MacGyver (2016), the reboot of MacGyver (1985). And then there are the cross-overs with Magnum P.I. (2018), itself a reboot of Magnum p.i. (1980).
Phew. That is a lot of interconnected TV. But we’re not done yet! Oh no, there is more.
In 2015, Duane “Dog” Chapman appeared on Hawaii Five-0 as himself, thus establishing that he exists within the ‘verse. And in 2018 Mike Wolfe of American Pickers popped up on NCIS, adding his show in to the mix.
So many series tied together does of course bring up problems. Aside from many actors being reused across the various shows (Terry O’Quinn, Sean Murray, Michael Bellisario, Dean Stockwell, and Zoe McLellan being the most obvious), there are the various references made to the other shows. DiNozzo in NCIS was a big Magnum p.i. fan, and Gibbs mentioned Hawaii Five-O once.
Then there is DiNozzo naming Bellisario, and being a Quantum Leap fan but not recognising that Dwayne Pride is Scott Bakula.
This does bring up the interesting possibilty that Dwayne is actually Sam Beckett on an extended leap, but temporarily unaware of his circumstances, not being in contact with Al. If you look at S05E01 “See You Soon” in a certain way, it could be seen to be his daughter Samantha (or even Ziggy) trying to bring Sam back into the QL program.
If Pride exited the series in a blue halo with the appropriate sound effects it would be a better end than his last leap: the one where he spent four years as a ship captain that may or not have been a computer simulation run by a fat man with a beard.
Sorry, I got carried away again.
At the end of NCIS: Los Angeles Season 10, the 14 year cliffhanger from JAG was revealed (look elsewhere for spoilers), but that doesn’t really change the tree – JAG characters had already appeared in NCIS: LA.
However, a chance viewing of an old Magnum p.i. (1980) episode showed a cross-over with Simon and Simon (1981). More research turned up a Magnum/Murder, She Wrote (1984) crossover too.
There were rumours of other cross-overs and spin-offs, and an unlikely one turned up the from the Dog show Dog and Beth: On the Hunt (2013).
No doubt purists would disagree with the inclusion of the “reality shows”, but they stay for now.
As of Monday, June 10th 2019 this is up to date. If things change, I will try to keep up with the changes.