Vince Bianchi a mob boss from the '70s is found buried under concrete after he was murdered in 1978.
Vince's best friend, Harold may hold the key to finding the murderer but he has delusions that it is still 1978. In order to make him comfortable enough to share his information, the 12th precinct transforms itself into the '70s with Martha's help.
Harold tries to kill Frank Russo, another ex-mob boss when he believes Frank murdered Vince. That's when Beckett realizes he's no longer delusional. But Frank didn't kill Vince.
Frank's wife, Maria is the sister to another mob family. Vince was going to propose to her but then backed out and she killed him in a fit of rage.
Vince backed out of the marriage proposal because he and Harold were lovers and his heart wouldn't let him marry someone else, even if it was for the good of the family.