Supergirl has building toward a Green Martian vs. White Martian showdown for a while now, and on Monday's episode, cheekily titled "The Martian Chronicles," it finally happened when M'gann's (Sharon Leal) former mate and another White Martian arrived on Earth to hunt her down for being a traitor. Unfortunately, the episode also revealed a near-fatal flaw in The CW superhero series, which is that the security at the DEO is horrendous.

Honestly, how many times has someone breezed into or breached what should be a building with almost annoyingly intense security protocols? It's a top secret organization and the building is apparently (and conveniently) powered by a nuclear reactor, for crying out loud! You'd think someone somewhere would have designed a security system that involves more than what I can only imagine is a simple sign-in sheet presided over by Mike, the night security guard with a soft spot for pastries. It's true that when someone can shape shift, like the White Martians could here, that it's probably going to be a bit harder to identify friend from foe, but come on, you're the DEO! You deal with aliens on a daily basis. Step up your game!

Hercules star Kevin Sorbo is heading to Supergirl

Melissa Benoist, <em>Supergirl</em>Melissa Benoist, Supergirl

The arrival of two White Martians hunting M'gann set up an intense hour — well, as intense as an hour can be when you know Supergirl and Co. are inevitably going to save the day — in which the enemy could be anyone and also everyone. It's a tried and true storytelling dilemma, one that ratchets up the tension and usually allows for some deep soul searching, and the same was true here.

As the team split up to track down what they initially believed to be just one White Martian impersonating Winn (Jeremy Jordan), J'onn (David Harewood) confessed that M'gann's presence in his life filled the hole in his heart (aww!) and she admitted that she was brave because he showed her how to be (double aww!). In the end, she chose to return to Mars seek out voices like hers, to stand up and fight against the White Martians who believed they'd created the perfect paradise by obliterating the Green Martians for being what they perceived to be inferior.

Teri Hatcher joins Supergirl as mysterious big bad

But while J'onn And M'gann were getting all emotional, Kara (Melissa Benoist) and who we thought was Alex (Chyler Leigh) were engaged in a heart to heart about Kara's fear of being abandoned by her sister now that she's dating Maggie (Floriana Lima). While it may have seemed like a bit of an overreaction on Kara's part, she's forgiven because it's never acceptable for anyone to ditch someone, whether it's on their Earth birthday or simply Taco Tuesday, for a Barenaked Ladies concert in 2017. It's just not. Alex should be embarrassed.

Of course the two sisters eventually made up before the hour was over, with Alex even pushing Kara to be honest with herself regarding her recent decision to push Mon-El (Chris Wood) away. Unfortunately, before Kara could rectify her obvious mistake, Mon-El found someone else in the form of Kara's replacement at CatCo, and well, I don't know what else anyone expected. Nothing is ever easy on Supergirl. Let's just hope the series doesn't drag this emotional torture out for too long.

Supergirl airs Mondays at 8/7c on The CW.

(Full disclosure: is owned by CBS, one of The CW's parent companies.)