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20p Archivescurtain has come down on the third season of Only Murders in the Building, and with it we have a solution to the mystery of who killed Ben Glenroy(Paul Rudd). It turns out, our amateur sleuthing team here at Mashable got a lot of things right. But we did get some deductions wrong — and got hooked by one big red herring. 

Let's break "Opening Night" down, spoilers and all. 

Who killed Ben? The truth was revealed! 

Back on Season 3, episode 7,  "CoBro," there was a lot of drama going on. Charles (Steve Martin) had quit Death Rattle Dazzleover the impossible "Pickwick Triplets" patter song.So Oliver (Martin Short) recast the role with Broadway legend Matthew Broderick. But this wasn't the only cameo from the hit musical The Producers, as the episode also included an appearance from playwright/lyricist Mel Brooks. 

At the time, Mashable's own Sam Haysom suggested that these two cameos combined could mean the killer was Death Rattle Dazzle's producers. After all, the plotline of The Producersis that two crooked creatives are making a purposefully bad play to collect insurance money. Could mother-son duo Donna (Linda Emond) and Cliff DeMeo (Wesley Taylor) be following Brooks' play's book? 

We got it half-right. Donna and Cliff are responsible for killing Ben. But they didn't do it for the insurance, and they were not in cahoots! 

Why did Donna and Cliff kill Ben? 

In the penultimate episode, "Thirty,"Mabel (Selena Gomez), Charles, and Oliver determined Ben had been felled by his weakness, a cookie, which had been laced with rat poison at the theater. They further deduced it was Donna DeMeo who did it — after she had seen theater critic Maxine's (Noma Dumezweni) would-be review, which pointed to Ben as the show's worst element. Donna bought the scrumptious cookies from Broadway mainstay Schmackary's,dusted one with the rat poison in K.T.'s office, and laid it in Ben's dressing room, where she knew he couldn't resist. And for good measure, she urged the frantic leading man to "be nice to Ben," urging him to take a fateful (and potentially fatal) bite. 

However, in the finale "Opening Night," Donna confesses to the crime-solving trio that while she poisoned Ben, her goal wasn't death. 

"Maxine's review would have killed the show," she says directly into documentary filmmaker Tobert's (Jesse Williams) boom mic, "So I bought us some time. I wasn't trying to kill Ben, I just wanted to knock him on his ass. Of course, it's hard to know the exact non-lethal amount of rat poison to give a man. How many rats is one Ben Glenroy?"

But Ben didn't die at the theater; he just collapsed on stage there. Instead, Ben died at the Arconia after ducking out of Oliver's opening night party to take a curious phone call. Last week, we speculated who might have been on the line. This week, we learn it was Ben's doctor, informing him that he'd been poisoned. Ben figured this out right in front of Cliff, Donna's beloved son, whose reputation she was trying to protect by getting Ben and his wooden performance off the stage. Death Rattle was to be Cliff's first production, and as Donna's greatest pride and joy, she wanted it to be the best it could be. But that loyalty goes both ways. 

Outside of Oliver's apartment, Cliff came out to find Ben, reeling from the news he'd been poisoned. As the movie star had been fasting, it was pretty easy to deduce the source of the toxin. "That's all I ate — one fucking Schmackary!"Ben exclaimed. From there, he works out Donna's dastardly deed. In response, Cliff reasons, "Maybe she was protecting me. It's what she does." 

"She thought I was ruining her boy's play — her real star!" Ben shouts, "And so she tried to murder me!?" 

From there, the two argue, then grapple, Ben furious and still riling Cliff by calling him "boy." The first-time producer then shoved the "phony" star down the elevator shaft. "It was an accident, Mom, I swear," Cliff insists. 

As "For the Sake of a Child" plays from the murder mystery musical Death Rattle Dazzle, a flashback shows Cliff pitching Ben to his death. And with him went the handkerchief that proved a crucial clue. 

How did the handkerchief factor in as evidence in Ben Glenroy's murder? 

Early on, Mabel and Charles imagined the white handkerchief with red rattles on it that Ben had in hand when he died would be the key. They envisioned that Ben had grabbed it as he fell. While the hankies — a gift from Ben to the cast — were all accounted for, they hadn't considered those given to the producers.

• 'Only Murders in the Building' Season 3: Here's all the lyrics to the 'Pickwick Triplets' song
• Those 'Only Murders in the Building' episode 7 cameos may have given away who did it
• Every single 'Only Murders in the Building' Season 3 end credits Easter egg
• The cookies in 'Only Murders in the Building' are a sweet Broadway Easter egg
• The gang's back for 'Only Murders in the Building' Season 3 — here are the best ways to watch

In episode 7 "CoBro," Uma Heller (Jackie Hoffman) revealed she'd shoplifted the bloody memento and sold it to someone she assumed was a CoBro fan. But it was Ben's brother Dickie (Jeremy Shamos), who bought it because he thought he was protecting Loretta (Meryl Streep) — with whom he had a connection that could not yet be explained. In "Opening Night," Loretta revealed he'd given her the hankie, and when she shared this with the OMitB trio, they spotted Donna's lipstick amid the blood stains. However, a minor ritual on opening night revealed the handkerchief's true owner. 

Before the show began, Donna took Cliff's hankie from his breast pocket, blotted her lipstick, and then put it back. "One for the lips," she said, kissing his face. "And one on your heart." This is how Mabel realized it was Cliff, not Donna, who ultimately killed Ben. Donna's lipstick — but Cliff's kerchief! 

Curiously, a tale of mother and child being fiercely protective played out over and over this season. Loretta falsely confessed in "Sitzprobe" to save her secret son Dickie from suspicion. Donna poisoned Ben to save Cliff from bad reviews. Dickie hid evidence to protect his birth mother, Loretta. Cliff killed Ben to save his mother from prosecution. Even in Death Rattle Dazzle, the nanny (played by Loretta) kills the detective (played by Ben and Oliver) to protect the Pickwick triplets. And she does it by pitching him off the lighthouse! (How eerily prophetic!) 

Tobert is not guilty. 

Okay, so Tobert the lying documentarian with true-crime interests was a red herring. His crush on Mabel and wedging himself into the case may have been sincere (or a bit of career ambition), but it wasn't to cover up some murderous secret. His handwriting —which seemed like it would be a clue— meant nothing. On one hand, he was a great suspect. On the other hand, Only Murders in the Buildingalready had a love interest who proved to be the killer with Season 1. So, maybe it's best they didn't retrace that step.

However, Tobert could well factor into the newly announced Season 4. After all, he was at the Arconia when another murder was committed in the building….

How to watch: Season 3 of Only Murders in the Buildingis now streaming on Hulu.