Red Kryptonite arrived in National City on Monday night. It turns out, the diabolical creation of Maxwell Lord (Peter Facinelli) was actually a weapon for making the best episode ofSupergirl yet.

Kara (Melissa Benoist) becomes infected with the synthetic Kryptonite substance while assisting a firefighter stuck on a roof, but it doesn't kill her like organic Kryptonite would have. Instead, it alters her brain and removes Kara's uber-polite mental filter so that all of her deepest, darkest, and meanest thoughts come to the surface. The result? Every time she opens her mouth, she torpedoes another important relationship in her life.

The Flash to crossover with Supergirl

One of Supergirl's best assets is Benoist's ability to make Kara bubbly without being annoying. To the actress' credit, Kara's unwavering optimism and faith in the goodness in people has never felt forced. That's what makes it almost shocking that Benoist pulls off such a deliciously malicious version of Kara. While an evil Supergirl was bad news for National City, a darker version of Kara was actually a lot of fun to watch. (To quote Calista Flockhart's Cat Grant, "Brazen is a new color on you. I don't mind it, yet.") Let's be real, no one except for Winn (Jeremy Jordan) was mad when Kara finally stuck it to Siobhan (Italia Ricci).

However, Benoist's fun walk on the wild side was only topped by Kara's heartbreaking return to the light, and realization of all the damage she had caused. "It was kind of a challenge to play this character that I had spent 15 episodes playing [in] this very specific way, discovering she's so morally sound and intrinsically good. To take that and completely throw it away was liberating and exciting," Benoist explains to TVGuide.com. "The problem with Red Kryptonite is that it kind of just brings everything to the surface, so all of these things that go unsaid or feelings that she might have had that she pushes down, she lets it all out. She says some things she can't take back."

Mars attacks Supergirl and it's bad news for J'onn J'onzz

In other words, while it may not be Kara's fault that she was contaminated by the Red Kryptonite, she's still responsible for the fallout. That includes her now broken relationship with James Olsen (Mehcad Brooks), after Dark Kara told him exactly what she thinks of his ex, Lucy Lane (Jenna Dewan-Tatum). For any fans hoping that James and Lucy's breakup last week would be a chance for Kara and James to finally reveal their true feelings to each other, keep waiting. James isn't ready to hear Kara's apology and both of them need time to heal before testing the waters of a relationship.

But of all the people that felt the burn of Kara's unhinged candor, it was Hank (David Harewood) who lost the most to the effects of Red Kryptonite. An enraged Supergirl came within seconds of vaporizing Alex (Chyler Leigh) with her heat vision, and the only way to prevent this was for Hank to take her on as J'onn J'onzz. Hank's transformation into his true alien form meant giving up his life as Hank Henshaw and his position as head of the DEO. He now resides in a cell inside the military compound while the DEO waits for new management to weigh in on his fate. While J'onn embraced Dark Supergirl's advice to be who he really is, Kara's guilt over forcing him to potentially forsake the role that's given him the most purpose on Earth will undoubtedly change their dynamic going forward.

Everything to know about Supergirl's Martian Manhunter

The repercussions of everything that Kara and Supergirl did in this episode will be felt for weeks to come, which creates a compelling arc for the character going forward. The show made an admirably mature move in not having the Red Kryptonite contrive what Kara was saying. As ugly as everything she said and did was, it still came from inside of her and Kara will have to own that (and do some personal growth) to return to the righteous place that she was before.

This episode not only highlighted Benoist's range as an actress, but also proved that Kara is more than a cookie-cutter superhero. She's not innately better than the people she protects, or the audiences at home. She struggles with the same dark thoughts and weaknesses as everyone else, but makes a choice every day to have faith in people, and to be good. Her dark side makes her more relatable, but her ability to overcome it will make her a better superhero and an even more interesting (and inspiring) one to watch.

If Supergirl, which has seen a decline in ratings since its strong start in the fall, is still trying to find it's stride, then it should consider Monday's installment a beacon of light towards the right path.

Supergirl airs Mondays at 8/7c on CBS.

(Full disclosure: TVGuide.com is owned by CBS)