[This is a review of Supergirl season 2, episode 19. There will be SPOILERS.]

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Supergirl season 2 has introduced and followed a number of ongoing storylines, ranging from Kara Danvers' fight against the villainous organization Cadmus, general anti-alien hatred, the arrival of the Daxamite Mon-El, and her burgeoning career as a reporter for CatCo Magazine. Season 2 also saw more focus on romantic relationships as Supergirl followed Alex's coming out and newfound relationship with National City police detective Maggie Sawyer as well as Kara and Mon-El started dating in more recent episodes.

While Kara and Mon-El's relationship has been at the forefront recently, especially since the arrival of Daxam King Lar Gand and Queen Rhea, this week will see Alex and Maggie's relationship take center stage. The pair hit a road bump in Supergirl's Valentine's Day episode but has largely been strong ever since. Now, with Alex kidnapped, their relationship is tested once again.

In this week's Supergirl episode, 'Alex' -- written by Eric Carrasco and Greg Baldwin and directed by Rob Greenlea - Supergirl and Maggie must work together to save Alex after she's kidnapped by someone from Kara's past. However, with the kidnapper asking Supergirl to release a notorious criminal from prison, she and Maggie must find another way to save Alex. Meanwhile, Lena Luthor considers a proposal from Queen Rhea after the Daxamite arrived in her office at the end of last week's episode, 'Ace Reporter'.

Team Supergirl + Maggie

Beyond the moments in which Kara and Maggie have been spending time or working on defeating a villain with Alex, the pair haven't had too much interaction. For the most part, Alex - or, on rare occasions, someone else from the DEO - has acted as the go-between for Supergirl and local law enforcement. However, 'Alex' provides a deeper look into the dynamic between Supergirl and the National City police force, and it isn't as perfect as viewers may have thought.

In fact, Maggie pulls the curtain back a bit on exactly what the NCPD thinks of Supergirl and her vigilantism with the help of her alien superpowers. While Supergirl has certainly depicted the hero as flawed, the show largely focuses on characters who are either allied with Kara -- even Snapper Carr is, at the end of the day, begrudgingly thankful for both Kara and Supergirl - or those who are her enemies. However, Maggie finally shines some light on what certain people in National City think of her vigilantism, and it isn't all sunshine and puppies, but it does make for some compelling insight. Maggie reveals that criminals have begun using the "Supergirl defense" which helps their cases because Supergirl operates outside of the law, and sometimes injures those she apprehends.

This discussion comes about after Kara swooped into a hostage negotiation Maggie had been working on for 17 hours and uses her abilities to capture the criminals. Maggie is justifiably frustrated that Supergirl intervened in a situation the NCPD had been handling (for 17 hours, no less), and it causes an argument about their different styles of approaching a case. By no coincidence, this argument sets the stage for the main storyline of the episode: When Alex is kidnapped, both Kara and Maggie try to approach the problem their own ways, but it isn't until they work together that they are able to save the person they love.

'Alex' is an emotional, character driven episode and, as such, is one of Supergirl's strongest. The dynamic between Kara and Alex - played perfectly by Melissa Benoist and Chyler Leigh -- was an emotional throughline of season 1 and became even more important in season 2 with the departure of Cat Grant. Additionally, Alex's relationship with Maggie has also been significant throughout season 2, despite being mostly removed the Supergirl's titular heroine.

However, aside from moments in which the characters are all spending time together or instances in which Alex turned to her sister for relationship help, Supergirl hasn't truly addressed the changing dynamic between the Danvers' sisters with the addition of Maggie to the mix. Now, with Alex kidnapped and Kara and Maggie turning to each other for help, they butt heads but inevitably are able to work together. Of course, it's the process of the two learning to work together that makes 'Alex' so compelling, and the sometimes at odds points of view of Kara and Maggie are played well by Benoist and Floriana Lima.

While both Benoist and Lima were strong standouts of 'Alex', Leigh's performance cannot be undersold. Despite spending the majority of the episode locked in a glass box or talking to the voices of either Kara or Maggie, Leigh delivers an exceptional performance - all while playing Alex as the capable DEO agent who manages to wring every second of time out of her situation and stay alive until she's rescued. And, of course, the moment in which Alex and Maggie say 'I love you' for the first time brings the emotional episode to a happy conclusion. On the whole, 'Alex' is a testament to the quality of the actors, and the Supergirl writing team, providing a thrilling and emotionally wrought hour of drama - one that just so happens to be on a superhero show.

Lena Makes A Deal With Rhea

Elsewhere in National City, Rhea attempts to partner with Lena and L Corp in order to build new technology. However, the Daxamite queen attempts to disguise herself as a human genius -- one Lena somehow hasn't heard of. The two manage to bond over dinner as they discuss their troubled relationships with their respective families. Rhea, of course, leaves out that the fact that the "horrible girl" who stole her son away from her is in fact Lena's best friend, Kara Danvers. But, their burgeoning partnership hits a snag when Lena figures out Rhea is an alien.

Prepared for everything -- and getting a streak of luck with Kara being preoccupied by Alex being kidnapped - Rhea plays on Lena's desire to distance herself from her family's anti-alien legacy. The two make a deal to go into business together, which largely works to set up the upcoming episodes. Supergirl season 2 has positioned Rhea as the major villain of its closing episodes, all the while teasing the possibility that Lena could turn on her friend at some point. It seemed the death of Jack Spheer in last week's episode could do just that, and to an extent it has since Lena is now working with Rhea.

Of course, how Rhea and Lena's partnership plays out in the final episodes of Supergirl season 2 remains to be seen. But with this particular step forward adding even more emotional stakes to the upcoming battle between Kara and Rhea, it's certain that the end of Supergirl season 2 will have plenty of impact not only on National City but on a number of Kara's relationships.

Next: How Supergirl Proved Fan Demand for Wonder Woman

Supergirl continues Monday, May 8 with ‘City of Lost Children’ at 8pm on The CW.