WARNING: The following article contains spoilers for "The Martian Chronicles," the latest episode of "Supergirl."

"The Martian Chronicles" takes its title from a Ray Bradbury short story collection of the same name, but if this episode of "Supergirl" has literary roots, they're not to be found in the science fiction section. Instead, episode writers Gabriel Llanas and Anna Musky-Goldwyn seem to have their cues from mystery writer Agatha Christie, whose locked-room mysteries have received dozens of adaptations over the years and inspired countless other novels, films and television episodes. It's a terrific formula: a crime occurs, and for some reason, none of the players can leave. A bridge washes out, there's a power outage, the doors or locked, they're already on a train -- you get the idea.

These stories work because they're hotbeds for tension, and that's certainly true of the centerpiece scene in "The Martian Chronicles," a roundtable so laced with paranoia that it seems designed to make viewers gnaw at their cuticles. Unfortunately, not all of the episode lives up to that standard, and nor does it meet the requirement most essential to telling a good mystery: it has to make sense. "The Martian Chronicles" has plenty going for it, including fight sequences as good as any in the series to date, but sense isn't one of those things.

As the episode starts, we leap right back into the Kara/Mon-El situation, this time at everyone's favorite alien-friendly watering hole. Over a nice, cool glass of tonic water -- Kara, come on, you can do better than that -- the two discuss the awkward fall-out of Mon-El's confession of like, and things only get more uncomfortable as Kara plants both of her steel feet inside her mouth. After a hurt Mon-El has walked away, things get worse for our heroine, as Alex comes over to tell her that whoops, she's not free to celebrate Kara's earth-birthday because Maggie has purchased them surprise Barenaked Ladies tickets. Kara's clearly not pleased, and just like that, two close relationships suddenly get rocky.

That makes it the perfect time for a genocide-loving maniac and terrible husband to turn up on earth. As teased in the previous episode's final moments, M'gann's being hunted by White Martians, who want to punish her for her actions when leaving Mars (they call them "war crimes," though she's clearly on the side of the angels here). J'onn, who's been following M'gann to protect her, leaps into action when the big white dude lands behind the bar, and while our two good Martians put up a hell of a fight, it's the arrival of the tonic-fueled Supergirl that sends the bad Martian whimpering away.

So who is this baddie, exactly? Turns he's named Armek, and he's M'gann's former mate (she later explains to the DEO team that it was a forced marriage). It's an interesting choice, as the language Armek uses when speaking to M'gann hints at the idea that she's somehow his property. It brings to mind situations of domestic violence, lending an even more upsetting air to the whole relationship. Armek gives M'gann a time by which she must surrender, and tells her that if she doesn't, he'll dearly enjoy killing all her new earth friends.

RELATED: Flash/Supergirl Crossover Set Photos Offer First Look at Music Meister

Of course, J'onn's not having that, and neither is Kara. The whole DEO gang -- including Alex, who rushed off to apologize to Kara before meeting Maggie at the concert, and a still-pissed-about-Guardian-stuff Winn -- gathers in the control center to talk with J'onn and M'gann about the situation. Just then, a different M'gann walks in and all hell breaks loose. Director David McWhirter stages the hell out of this fight, a chaotic affair that involves loads of flashing lights and a truly epic television explosion. When the dust settles, one of the two M'ganns is gone, J'onn locks down the building and we're in Agatha Christie territory.

That's not what happens, though, because from here on out the story leaves logic far, far behind. Since they're locked down, J'onn knows the Martian must be in the building. Another team member points out that, in theory, any one of them could be the Martian thanks to the shapeshifting and all. That they were all in the room when both the M'ganns were doesn't seem to occur to anyone, nor do they recognize that there would have to be an extra body somewhere in the room. It just doesn't make a lick of sense.

There's no way to parse this out without revealing the secret, so if you're reading this for the story, you may want to skip this next paragraph and come back. Still here? Good. Onward.

So there are two Martians in the building, but M'gann, fake M'gann, Alex and Winn are all in the same room at the same time. The fight happens, fake M'gann seems to disappear, and J'onn locks down the room. So at what point does fake M'gann become either Alex or Winn? There's no way they could have been spirited away to get gooey-ed onto a wall in that brief blackout, and even if they could go missing that quick, we'd have seen the Martian running back in. The only way any of this makes sense is if there's a third Martian on the loose -- fake M'gann, who disappears, and fake Alex and fake Winn, who were in the room the whole time -- but as M'gann's left to fight the good fight on Mars, that seems an unlikely twist.

While storytelling fundamentals may have flown the coop, the "Supergirl" writers room still knows how to craft a great scene, and the literal holding of hands to fire as a metaphorical way of holding one's feet to the flames makes for great drama. Best of all, as it's revealed that Winn's not really Winn, is the chance to see Jeremy Jordan nibble at the scenery as a very bad guy, complete with weaponized burners, a table-kicking and some truly evil grins. After he bolts, the team splits up to find both the real and fake Winns, as the former is needed to stop the building's nuclear reactor (whaaaat?) from melting down.

M'gann and J'onn and Alex and Kara have heart-to-hearts as they search, while the other team members are presumably off sniping at each other about all the gun-pointing they were doing. M'gann and J'onn declare their profound affection for each other -- it's a conversation that's clearly about an attachment so significant that to dismiss it as romance would be wrong -- while Kara and Alex unpack the new dynamic they're staring down now that Maggie and Alex are an item. Just when things seem calm, our good Martians find the unconscious forms of real Winn and -- gasp -- real Alex, leading J'onn to warn Kara just as fake Alex prepares to beat the holy crap out of her sister.

The good guys win, of course, with Winn furiously attempting to ignore an alien battle as he tries to save the city and defeat the evil genius work of his own fake self. Alex encourages Kara to ask herself why she put this much on her birthday -- was she denying some other feelings, perhaps about a certain bartender? -- and M'gann tells J'onn she's decided to return home to encourage other good White Martians to come out of the shadows. We leave Kara with Mon-El only to discover she's missed the boat, as "Mike," glasses and all, heads out on a date with Miss Tessmacher. Given that next week's a Luthor-centric outing, perhaps that new relationship's got an expiration date.

Oh, and those CW-specific "LEGO Batman" commercials? Killer.

Airing Mondays at 8 p.m. ET/PT on The CW, “Supergirl” stars Melissa Benoist, Mehcad Brooks, Chyler Leigh, Jeremy Jordan, David Harewood and Chris Wood.