It seems that every cop, detective or medical examiner show had at least one episode about boxing. I remember seeing this on "Hawaii Five-O". "Cannon", "Quincy" and many other similar shows. So, it's not surprising that "Mannix" eventually dealt with this locale.
The show begins with a retired and punchy ex-boxer, Sonny, beating the crap out of a gambler who was trying to fix a fight. At some point, SOMEONE shoots the gambler dead--and Sonny is seen standing there holding the gun. Did he do it? And, if he did, does he even remember thanks to all the poundings he took to his head over the years? Well, he isn't sure--and assumes he'll go to jail for it, so Sonny runs. Mannix's job is to find him and convince him to give himself up-- especially because soon some goons are looking to kill Sonny and/or Mannix.
This is generally a very good episode of the series. My only complaint is the lady who was singing in the bar. She was, to put it mildly, pretty bad. While a very pretty actress and good otherwise, they asked a non-singer to sing and it showed. Still, this is a minor quibble in an otherwise decent program.