It's technically a follow-up with different characters. They mention the Alonzo Harris (Denzel) scandal in the first few minutes to set up that the show takes place in the same universe years later.
shareyeah... the idea is that Alonzo's (Denzel) actions in the Film caused 'blowback' with many of his arrests being voided...
the LAPD sees Frank (Paxton) as causing a similar problem if when his actions come to light; so they assign someone to work undercover to track his actions so that they can 'backstop' his collars (double up on 'real' evidence to over compensate for any stains his name puts on the case)
it's a bit contrived... and really if I had to guess it's the result of some one pitching the show, and someone saying "hey that sounds like Training Day" so they contacted the people to get the rights, rather than risk being sued for copyright infringement
I get it.
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