In “Let This Mother Out,” we find the Charmed Ones right where we left them last week, face-to-face with Harry just seconds after the Ouija board — sorry, spirit board — warned them not to trust the Whitelighter. But it turns out, his ominous arrival was just the result of him being accidentally summoned to the attic by Maggie. However, he takes the time to remind the sisters that since the underworld now knows about them and their powers, they need to be on the lookout for strange, potentially demonic occurrences. Spotting the spirit board, he also adds that it is high-key off limits as they’re “notoriously vulnerable to tricksters,” before disappearing.
The sisters are left trying to figure out who to trust, leading to some serious tension between Mel and Macy. Mel is convinced they should trust that the spirit reaching out to them is really their mom, but the rational Macy isn’t convinced. Before blindly trusting the spirit board, she proposes giving Harry a truth serum described in the Book of Shadows to learn whether or not he has ulterior motives. The two argue over their preferred next steps, with Mel taking some serious jabs at Macy, who is already feeling like an outsider, being the new sister in the house. Mel tries to pull a “majority rules” stunt on her, but Maggie sides with their older sister in wanting to test the truth serum idea. Meanwhile, to avoid accidentally summoning their Whitelighter, Maggie is insistent on referring to Harry as Meghan Markle for obvious reasons, and also because this show is nothing if not culturally relevant.
Off to stand vigil for the still-comatose Angela Wu, Maggie is still learning to control her powers, finding that unfortunately, even gloves won’t prevent her from hearing the thoughts of everyone she touches. She also unexpectedly discovers her powers work on the comatose, hearing some very haunted screams from inside the mind of the unconscious Angela. Later, her “just friends” hookup with Brian is also marred by her ability to read his thoughts. All in all, Mags is feeling very frustrated with her power.
Macy runs into Galvin at the lab and clears the air about the whole beer bottle incident from last week. After they make up, Galvin fills her on an incident that happened earlier in the lab involving a custodian who claims she was attacked by a ball of black goo. While the incident is being chalked up to the custodian having a nervous breakdown, Macy knows something potentially demonic is at play and manages to find and extract some residue from the goo to give to Harry, barring results of the truth serum. She goes to visit the custodian in the hospital and finds Harry already there. He claims to just be wiping her memory, but Macy is very suspicious. Later, Galvin tells her that the custodian is dead and it seems things are not looking good for Harry right now.
Meanwhile, Mel is casually in the attic when the spirit board begins to move. The board is able to convince Maggie that their mom is using it to communicate with them by answering questions she believes only her mom would know. However, just as they’re getting ready to connect, the board reaches out and grabs Maggie — which everyone who has ever seen a horror movie involving an Ouija board knows is bad news. Walking in on the sitch, Macy telekinetically sends the board across the room, accidentally breaking it. Upset, Mel accuses her of not wanting the spirit to be their mom, prompting Macy to let her have it. She lays it all out there, telling Mel and Maggie that she thinks she’s more objective to the potential danger in trusting the spirit board as she didn’t experience the loss of Marisol in the same way they did. She adds that while the three of them may be the Charmed Ones, she’s still very much the outsider in the sisterhood.
The fight is interrupted by the arrival of Niko, who is officially back together with Mel. Macy storms off for her truth serum tea date with Harry and in a classic accidental thermos switch, takes off with NIko’s thermos, leaving Niko with Macy’s truth serum. Having accidentally consumed the serum, Niko begins to drop truth bombs on everyone, including a suspect, her coworkers, and Mel.
Maggie and Mel conspire to fix the spirit board using magic and while Mel is dealing with the Niko truth situation, Maggie fixes it, only to have the spirit reach out to her and reveal there’s a way to release it from the board. It’s all deeply suspicious, and as if I wasn’t already convinced that spirit is not their mom, I am now. But nope, Mel and Maggie have a full on seance and a being that certainly looks like their mom floats out of the board. After a brief reunion, spirit Marisol reiterates that Harry is not to be trusted, only now she adds that he was the one who killed her and he wants to steal the Charmed Ones’ powers.
Meanwhile, at their painful tea date, Harry grows deeply suspicious of Macy’s incessant questions about British life as she waits for the serum to kick in. Using his Whitelighter magic, he appears with Macy into the girls’ attic and comes face-to-face with spirit Marisol. However, before he can do anything, Mel knocks him out with the Book of Shadows.
After reuniting with Macy, who is now fully convinced the spirit is their mother, spirit Marisol tells the girls they need to find the Prism of Souls in order to protect themselves against Harry. However, Maggie senses something isn’t right as she can’t hear her mother’s thoughts. While spirit Marisol explains it’s because she’s not a person but a spirit, Maggie isn’t convinced and says as much to her sisters during their mission to collect the prism, which is being housed inside an old mirror in Harry’s — formerly their mother’s — office at the university. She’s overruled and the trio grab the prism and return home, only in to find spirit Marisol and Harry in mid-combat.
Spirit Marisol begs for their help, while Harry tells the sisters that she’s not actually their mom but an “imposter demon” who wants to steal their powers, adding that it knows the answers to their personal questions because it can get inside their heads. Luckily for Harry, Maggie believes him, citing her intuition. Spirit Marisol then begins pleading with Mel to believe her, but after implying that Mel was her favorite, Mel knows spirit Marisol is not their mom and stabs it with a knife. The wounded demon makes a play after Maggie, who is holding the prism, but when Harry explains that the demon will be destroyed if it sees its true form, Maggie’s able to get rid of it with the help of her cell phone’s handy selfie mode — and her selfie lighting case.
Having vanquished the demon, the sisters make amends with Harry — who, despite his penchant for speaking in riddles, has now earned their trust. Macy remembers her sample of the black goo and hands it to Harry, who immediately knows it’s something evil and goes off to see what he can find out.
Alone, the sisters set some ground rules, agreeing that majority rules is a terrible system. Maggie, feeling assertive, also takes the time to dole out some advice to her older sisters and the three each go off to have necessary conversations: Once more, Macy apologizes to Galvin for airing her frustrations with her sisters at him; Mel forgives Niko, who while on truth serum, revealed she hooked up with her ex just before the two got back together; and Maggie ends things with Brian for good.
The sisters are bonding as they move Macy into their mother’s old room, when Harry reports back, revealing that things are very bad for the Charmed Ones. Killing a demon in the form of their dead mom may have been the traumatizing “demon of the week” but these ladies have bigger fish to fry…as the season’s Big Bad may have just revealed itself. Harry tells the sisters that the black goo is actually the primordial form of the Harbinger of Hell. As if the name itself doesn’t signal impending doom, he reveals that its arrival marks the third phase of the prophecy — the first two, as explained in the pilot, being the worst of men rising to power (a.k.a. Donald Trump becoming president) and the murders of their mom and other powerful witches. In short, the apocalypse is coming and it’s up to the Charmed Ones to prevent it.
Harry adds that once the harbinger finds its human vessel, “everything’s going to change” — presumably for the worse. Naturally, the harbinger has found its human vessel in the newly awake Angela Wu.