[This is a review of Supergirl season 2, episode 20. There will be SPOILERS.]

After season 1 of Supergirl split its focus between Kara Danvers' job working for Cat Grant and helping out the DEO, season 2 has dived more into the Girl of Steel's superhero efforts. As such, there have been plenty more aliens introduced throughout the course of season 2, including the Daxam prince Mon-El, who has become a member of the DEO team and Kara's latest love interest, and Winn's new girlfriend Lyra. The main villains of Supergirl season 2 have also been alien-related, ranging from the anti-alien organization Cadmus to the recent arrival of Daxam's queen and Mon-El's mother, Rhea (Teri Hatcher).

In recent Supergirl episodes, the show has been gearing up for the season 2 finale - which will be a star-studded affair with the returns of Kara's cousin Superman (Tyler Hoechlin) and her former mentor Cat Grant (Calista Flockhart), as well as the arrival of General Zod (Mark Gibbon). With Lena Luthor joining forces with Rhea on a new technological endeavor, the Daxam queen is building up her efforts to fight against the Girl of Steel. But, before Kara and Rhea face off in the season's climactic battle, Supergirl takes a week to deal with a different kind of alien problem.

In this week's episode, 'City of Lost Children' - directed by Ben Bray with a story by Robert Rovner and teleplay by Gabriel Llanas and Anna Musky-Goldwyn - National City endures new alien attacks. However, Kara and the DEO learn they're the result of a Phorian, a peaceful alien race; when Guardian goes to confront the alien perpetrator, James finds a child named Marcus (Lonnie Chavis). Elsewhere, Rhea moves her plan forward.

A Hero Without The Mask

Supergirl City of Lost Children Marcus

The main storyline of this week's Supergirl episode focuses on James and his continued attempts to protect National City as the vigilante Guardian. The episode opens with Guardian saving a woman from being mugged, and she's just as afraid of her would-be savior as the men who would have robbed her. The encounter disheartens James, who tells Winn he has seen plenty of these reactions to Guardian whereas Kara is beloved as Supergirl. James' dejected mood is only exacerbated when a Phorian attacks a street market in National City and Supergirl swoops in the save the day. Then, though James attempts to help the DEO investigate the attack, J'onn J'onzz simply shuts him down.

However, when Guardian receives a tip about the home address of a Phorian, James goes to investigate and manages to form a connection with the young Phorian named Marcus - whose mother attacked the market and since went missing. James forms this connection to Marcus by taking off his Guardian helmet - a physical representation for James' entire journey in 'City of Lost Children'. Although James set out as Guardian with intentions to help the people of National City, and he's managed to do so, he hasn't necessarily found his purpose as a hero. Through connecting with Marcus and taking off his Guardian suit, James is able to prevent a massive Phorian attack simply by being himself.

Supergirl hasn't given much time to Guardian in recent episodes, largely utilizing him as a support player for the DEO team when they're in a little over their heads. But 'City of Lost Children' manages to catch up with James and advance his transformation into a true hero in a compelling way. Of course, the storyline of a street vigilante learning to ground himself by forming connections to others isn't necessarily a new arc for a superhero TV series (and has, in fact, been covered extensively on Arrow), but it's climactic moment is true to Supergirl. At the end of the day, the series is about heroes whose true power lies in their hearts, not in their superpowers or abilities - and that's exactly what James re-learns about himself in 'City of Lost Children'.

Rhea's Endgame Revealed

Supergirl City of Lost Children Rhea Mon-El

The secondary storyline in 'City of Lost Children' sees Rhea moving forward with her plan, using Lena to build what's essentially a teleporter/portal. Although Rhea tells Lena it's meant to transport her home and improve life on Earth, the Daxam queen is manipulating Lena for her help. Rhea is an expert at manipulating Lena and keeping their work together entirely separate from Lena's friendship with Kara - though that may be more luck than anything else. Since viewers know not to trust Rhea, seeing her interact with Lena is both compelling heartbreaking, especially given Lena's contentious history with her own mother, the Luthor family, and the recent loss of Jack Spheer.

Still, the emotional core of this particular arc reaches its climactic point when Supergirl, Mon-El, and Martian Manhunter arrive to confront Rhea, and Mon-El pulls a gun on his mother. Earlier in the episode, Mon-El struggled with his residual love toward Rhea after seeing her on the street and thinking it was just a mistake. He also reveals that on Daxam he would have avoided dealing with his emotions, but he won't do that now because of who he's become on Earth. At the end of 'City of Lost Children', his emotions come to a head when he threatens Rhea, but is unable to pull the trigger because she's still his mother - and because he's become a hero.

Since Mon-El is unable to defeat Rhea, she is able to move forward with her plan successfully. With the portal open, Supergirl and Martian Manhunter down for the count, and Mon-El unable to stop her, Rhea welcomes dozens of Daxam ships to Earth. They swarm over National City and Rhea finally reveals her endgame: to establish New Daxam on Earth - presumably by conquering the whole planet, starting with Supergirl's home city.

Supergirl City of Lost Children Mon-El J'onn J'onzz

This final sequence certainly sets the stage for a massive final battle in Supergirl season 2 - as well as explains the need for Superman to return to the series and lend his cousin a hand. It's not clearly established exactly why Rhea wants to conquer Earth in honor of Daxam, especially considering how little she thinks of the planet and its inhabitants (aside from Lena). That said, it no doubt connects to her pride, her inability to let go of Mon-El, and her need to get revenge on Kara for "stealing" her son.

Of course, how this particular storyline plays out on Supergirl remains to be seen. Considering that an alien invasion falls in line with the stronger focus on aliens in season 2, Supergirl has the potential to wrap up a number of its overarching themes of the year in a climactic showdown between Kara and Rhea. The show has set up an exciting battle, which we'll see play out over the final two episodes of season 2.

Next: Supergirl ‘Persists’ as TV’s Most Political Superhero Show

Supergirl continues Monday, May 15 with ‘Resist’ at 8pm on The CW.