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Angourie Rice in HBO's 'Mare of Easttown'

This Theory About Siobhan On Mare Of Easttown Will Wreck You

I hate to say it, but the theory holds water.

Michele K. Short/HBO

Mare of Easttown hasn't suffered for lack of mysteries. Although the murder of Erin McMenamin has been the show's main story, there's also been a kidnapper and the uncertain paternity of Erin's son DJ, which have complicated the whodunit tale. With only one episode to go in the miniseries, fans have started to lean toward the idea that the father of Erin's baby isn't necessarily the same person who murdered her — which opens up the suspect pool significantly. Moreover, new clues in Episode 6 have fans focusing on a new possibility: Siobhan could be the killer on Mare Of Easttown. This theory may seem far-fetched, but the more fans learn about Mare's daughter, the more they're convinced.

Warning: Spoilers for Mare of Easttown Season 1, Episode 6 follow. Since the series' debut, Siobhan has been the one person in Mare's world who has her life together. She's the good kid, the one with a future that's going to take her far away from Easttown. She even dumped her girlfriend Becca for Anne, a radio DJ who is a loving and supportive partner.

But secretly, Siobhan is a mess. As fans learn in Episode 6, she found Kevin's dead body swinging from the attic beams. Mare sent her home, thinking her son had just snuck in to steal stuff. The trauma of that day has never been discussed.

Moreover, despite Siobhan's wide-open future, she's not so eager to leave Easttown. She drags her feet at applying to schools and hesitates to accept anything that would take her away from her family.

Michele K. Short/HBO

That Siobhan is not OK became even more evident in Episode 6, after Anne gently pushed her to consider perhaps leaving Easttown. Siobhan got high and drunk and started acting obsessive toward Anne. Viewers couldn’t help but notice the triggering event was Siobhan watching a YouTube memorial video for Kevin. In it, he says he’s performing a song about his “first love.”

But who was Kevin’s first love? Kevin and Carrie had Drew together. But Carrie doesn’t seem to have any illusions that they were more than two people who accidentally had a kid. Indeed, no one treats her like she’s someone Kevin cared profoundly for. Could Erin have been Kevin’s real first love?

Plus, and this is crucial, there’s the murder weapon. Episode 6 opened with the forensics of the gun that killed Erin. The rifling (spiral grooves inside the gun barrel) runs leftward. As the expert explains to Mare, that’s old-school style. Her father, Easttown’s sheriff before her, probably had a gun just like this.

Michele K. Short/HBO

So... where is his old gun, then? Mare hasn’t said, but fans are guessing it’s up in the attic — where Mare has not gone since Kevin’s death. But Siobhan may very well have and have found the gun while she was up there, and went on to use it.

Does this seem far-fetched? Maybe, but consider John and Billy. It was evident in Episode 6 that John was playing Lori to get Mare out to the fishing spot and setting up Billy to die there, bringing the case to a close. Most fans assume John is covering up for being DJ’s father and Frank’s diaper delivery was because he knew the truth.

But what if it’s not? What if Frank delivered the diapers because he knew DJ was secretly his grandson? What if John and Billy saw Siobhan kill Erin and John is doing what it takes to keep Mare from knowing the truth?

This could all come down to the subject of the photo Jess brought to the police. If Kevin’s in it, all signs point to Siobhan being the one to kill Erin as revenge for Kevin’s death.

Mare of Easttown concludes on Sunday, May 30, on HBO.