Trama

In the season finale, the trio races toward a resolution while their own lives are put in danger.

Having concluded that Jan may be the killer, Mabel and Oliver race to warn Charles. When there's no answer at his door, Oliver tries to break down the door and fails hilariously. Charles arrives from the symphony. Oliver and Mabel lay out their theory, but Charles rejects it, choosing to side with Jan and saying that he doesn't want to be part of their crazy theories. Mabel asks Charles to look at the evidence and she and Oliver leave. Realizing that Jan's apartment will be empty since she's at the symphony, the two of them decide to search it.

Jan arrives at Charles' apartment and apologizes for lying about being first chair. She and Charles sit and have a drink. She sees the bassoon cleaner on the table and asks about it. Charles tells her it was found in Tim's apartment. Jan wonders if Tim played the bassoon too and if the killer is targeting bassoonists. Then she asks if Charles thinks she's a suspect. Charles says that he doesn't but that Mabel and Oliver do. Jan and Charles confess their love for each other. Charles tells Jan that he'd love her even if she killed Tim. Jan comments that Charles isn't drinking as much as she is. He says he's not drinking at all, but merely taking stage sips, because the drink is poisoned.

In Jan's apartment, Oliver and Mabel find a box labeled "Jan's toxins" and realize that they were right: Jan poisoned Tim Kono. In Charles' apartment, Jan admits to killing Tim. She had met Tim in the elevator and had begun a sexual relationship with him since she is attracted to "lonely men with a big age difference" with her. The night of the murder, she had lured Tim to her place for a last drink, which she had poisoned. It was Jan's trash which Mabel, Oliver and Charles had seen Tim carrying that night. Charles realizes that Jan's bassoon had sounded different that night and that she must have used a recording to help give herself an alibi. Jan asks how Charles figured it out and he points out that the killer's handwriting matched that on a note she had left for him. Charles begins slurring his words. Jan says that she didn't poison the drink this time, but rather Charle's handkerchief. As Charles collapses on the floor, Jan talks about returning to Tim's apartment and shooting him as he lay dying from the poison. She says that this time she's going to do something more spectacular and opens the gas valve on the fireplace to flood the room with gas. As Mabel and Oliver head to Charles' apartment they encounter a tenant who complains of feeling dizzy. They realize that Jan intends to destroy the building to cover her tracks.

After Jan leaves, Charles rises from the floor, having exaggerated how incapacitated he is. Extremely woozy, Charles crawls to the couch and removes his phone from under a pillow. He had been taping the entire conversation, and has Jan's confession. Charles stumbles out of the apartment in an attempt to make it to safety and manages to get on the elevator. There he encounters several other tenants including Bunny, Dr Stanley, and Howard, but since they all hate him, they merely assume he's drunk and do nothing. Lester the doorman, assuming the same thing, but more solicitous of Charles' welfare, puts him back on the elevator and sends it up to his floor. Mabel and Oliver find him there and load the now immobile Charles into Oliver's dog stroller.

Traveling to the basement, they discover that Jan has rigged the gas system to explode and dismantle her changes. Jan arrives and holds Oliver and Charles at gunpoint. Charles stands up and delivers a moving and lucid monologue about his friendship with Mabel and Oliver. This turns out to be all in his mind, as Charles is still incapacitated and his speech is gibberish. Oliver rams Jan with the stroller and Mabel knocks her out. The authorities arrive and arrest Jan. Having saved everyone in the building from death, and caught the murderer of Tim Kono, the three friends enjoy renewed popularity among the tenants. Bunny arrives and tells them that their evictions will be rescinded, but promises to evict Oliver sooner or later. Mabel calls Bunny a "cranky bitch".

Some time later, the trio finishes the final episode of Only Murders in the Building. They celebrate with drinks on the building's roof. Mabel excuses herself and heads downstairs. As Charles and Oliver discuss their next move they hear sirens approaching the building. They get a warning text from an unknown number and race downstairs to find Mabel. They discover Mabel kneeling over a dead body in a tie-dyed "Only Murders in the Building" hoodie, an enormous bloodstain on her shirt. Mabel tells them "It's not what you think!" Rolling the body over, she shows them that it's Bunny and says that the woman stumbled into her apartment already dying. Oliver identifies the apparent murder weapon as one of Mabel's knitting needles. Just then, the police break in, holding all three at gunpoint. The police arrest the friends and lead them out of the building, past the scornful residents, a worried Oscar, and podcast host Cinda Canning. Cinda's assistant, who is taping audio of the events, says "Coming next fall, 'Only Murderers in the building'." Cinda steals the idea for her own.