A show-and-tell presentation of the flower-filled tunnel of love Martha envisions for Castle and Beckett's wedding serves as a precursor to the headache that comes with another pesky murder. The victim's time of death was over three decades ago. Castle realizes the decomposed corpse is missing mafia man Vince Bianchi. The guy pulled a Jimmy Hoffa-like disappearance back in 1978. Suspects in the case include guys with names like "Louie the Lip" and "Mickey the Blade." A member of Vince's own crime family, Frank Russo, also had motive.
Russo points Castle and Beckett to the victim's consigliore, Harold Leon. The guy's a little off. He suffers from something called pathological grieving. It all started back when Vince went missing. Harold took it so hard that he convinced himself that it's still 1978. His house, car and wardrobe are a groovy as can be. Harold isn't ready to believe that a woman could be a lead detective on a homicide case, so he directs all his relevant comments to the Captain. No, Daryl Dragon is not in the house. Captain is what Harold calls Castle as he reveals that he knows exactly who killed Vince Bianchi.
Castle believes they need to play into Harold's '70s delusion to get answers. The plan is to use the rubber-molded body Lanie created to show a more recently-deceased Vince. They also need to make everyone around look super groovy. Lanie does her part by dressing up in her old Foxy Brown Halloween costume. Harold sees the body of his old friend along with a few of his personal belongings. It finally hits him that Vince is gone. There's not much time for mourning when a man dressed in surgical garb opens fire in the morgue. No one is hurt, but the bad guy gets away.
Harold is willing to spill details on what he knows. He prefers to do so at the precinct, which must be transformed into a place where Baretta and Kojak could hang. Martha is in charge of the extreme precinct makeover. Alexis is a runaway in a halter top and Esposito and Ryan play the roles two past NYPD legends known as Snookie Watts and Ray Price. Starsky and Hutch have nothing on Snookie and Ray. An initially reluctant Ryan really gets into role. It's enough to get Harold to start talking about a club known as Glitterati which is still a '70s venue. Disco is not dead!
Captain Gates goes ballistic when she sees what Castle has done to her precinct which now houses the suspect in the morgue shooting. Robert Decker is a person of interest in six contract killings. Castle interrupts Beckett's interrogation of the guy when he realizes the victim likely went to Glitterati the night he was killed. The owner of the club just happens to be Frank Russo. Harold believes he's the man who killed his friend. He pulls a gun on Russo, but Espo/Snookie and Ryan/Ray stop him before he can pull the trigger.
Harold admits that he snapped out of his '70s delusion when he got shot at earlier. As for Russo, he says Vince had booked a private event at Glitterati. He thought he was getting ready to propose to some mystery woman. Actually, it wasn't a mystery woman at all. Harold is the one who held his heart. They were in love, but they knew they could never be together. So Vince was going to marry rival mob man Michael Carcano's sister to unite the two crime families. This mystery sis is none other than Frank Russo's current wife, Maria, who is also the killer. Case closed.
Harold is ready to start living in the present. Castle... not so much. He convinces the entire team to head down to Glitterati for a night of disco dancing. Snookie and Ray are there. Martha and Alexis, too. Even Captain Gates pops in to get down. Can you dig it? Castle and Beckett gave over seeing Harold sharing one last dance honoring the memory of his lost love, Vince, until the time comes when he catches him on the flip side.