WARNING: The following feature contains SPOILERS for the Supergirl season 4 finale.
The season 4 finale of Supergirl, "The Quest For Peace," masterfully resolved all of its outstanding storylines. As it did this, it also introduced two new threats to test Kara Zor-El and her allies in Supergirl season 5. The final scenes of the episode also set the stage for the upcoming Crisis on Infinite Earths event.
The action of Supergirl season 4 saw Kara Zor-El facing a menace she couldn't punch into submission - a rise in anti-alien sentiment following the attempts of various aliens to conquer the Earth. As the newly appointed President Phil Baker began to roll back protections for alien immigrants, an anti-alien militia known as the Children of Liberty enacted a campaign of harassment against those same immigrants. Both of these efforts were revealed to be the work of one man - Lex Luthor.
The depths of Luthor's plan and his ultimate goal were not revealed until "The Quest For Peace," but it posed little surprise that Luthor had his fingers in many pies. In addition to manipulating the American government and a domestic terrorist network, Luthor was also responsible for brainwashing a Supergirl clone dubbed the Red Daughter into an anti-American zealot and spearheading an attack on the United States through the rogue nation of Kaznia. In the end, however, the master manipulator was outmaneuvered twice - first by his younger sister, Lena Luthor, and then by a mysterious organization that had apparently been using scientist Eve Teschmacher to work Lex Luthor to their own ends. And in the background, unnoticed by anyone, the mysterious Monitor made two appearances in National City that seem to set up big problems for Supergirl and her friends in the year to come.
Lex Luthor's Ultimate Plan: Kill Superman
As complex as Lex Luthor's plotting was, his motivations were ultimately simple. Everything Luthor had done over the course of Supergirl season 4 was a means toward one end - killing Superman. Even setting himself up as America's greatest hero, finding wealth and acclaim through a new invention and effectively making himself the ruler of the United States through a puppet President was just icing on the cake compared to finally defeating his sworn enemy.
The penultimate episode of Supergirl season 4, "Red Dawn," revealed that Luthor had been using both President Baker and the Children of Liberty to capture alien test-subjects for his experiments. While Lex had researched ways to implant alien abilities into humans, the primary goal was to find a way to drain the bio-energy of aliens and use them as an energy source. This, in turn, was utilized to empower a larger version of CLAYMORE - a satellite-based defense system meant to destroy alien ships approaching Earth. The new CLAYMORE had the capacity to vaporize much larger targets - like the asteroid holding Argo City, the last Kryptonian colony in the universe.
Thankfully, Martian Manhunter and newbie superhero Dreamer were able to stop Luthor's plan. After being captured by the Children of Liberty and put to work in Luthor's refinery on Shelley Island, the two were able to organize a rebellion and deactivate the power dampeners keeping the alien captives subdued. With no time to hack the computers overseeing the power core of CLAYMORE, the two heroes used their considerable psychic energy to overload the system before Lex could fire the blast that would destroy Argo City and the vacationing Superman with it.
The Death Of Red Daughter
An angered Lex Luthor flew to the Shelley Island facility in his personal power armor, the Lexo-Suit, to find that his alien captives were fleeing and that Supergirl and the rest of her allies had arrived. Supergirl challenged Luthor to a one-on-one fight, revealing that she had an armored suit of her own that protected her from the Kryptonite radiation that the Lexo-Suit generated. Unfortunately, the suit wasn't quite strong enough to defend against every weapon Luthor had and he was eventually able to wear down the Girl of Steel.
The Red Daughter, whom Luthor had imprisoned in one of his energy-draining pods on Shelley Island after he no longer needed her for his plans, entered the battle and took a Kryptonite energy blast that was meant to finish Supergirl off. She died in Supergirl's arms, using her last breath to apologize for being wrong about her and urging her to continue to fight to protect her people. The Red Daughter's body then dissipated into a purple energy, which Supergirl absorbed into herself. This gave her the second wind she needed to fight back against Luthor and damage his armor enough to send him flying.
The Death of Lex Luthor
As the Lexo-Suit began to falter, Supergirl caught a falling Lex Luthor and urged him to stop struggling against her. Replying that he'd rather die than let a Kryptonian save him, Luthor released the connection between his gauntlet and the rest of the Lexo-Suit, apparently dying instantly as the Lexo-Suit exploded on impact with the ground below. Unsurprisingly, Luthor had one trick left up his sleeve and he was able to teleport away at the last second to a secret base under his childhood tree-house, where he found his sister Lena waiting for him.
Luthor wasn't surprised that Lena had anticipated where he would flee if his plans failed. He wasn't even surprised when she pulled a gun on him. Luthor was, however, quite stunned to find that he had finally pushed Lena far enough for her to decide that killing him was the only way to ensure the world's safety, as she shot him twice in the chest.
Lena Luthor Learns Kara's Secret Identity
Lex did win revenge of a sort, using his dying moments to show a series of videos to Lena. The video footage proved that her best friend, Kara Danvers, was Supergirl. More, Lex told Lena that all of the important people in her life - her friends, her boyfriend James Olsen and even her own mother, Lillian Luthor - had known Kara's secret and hidden it from Lena. With his final breath, Lex mockingly told Lena that while he might be dying "at least I lived without ever being a fool" whereas Lena would live on with no one she could truly trust.
This struck a heavy blow to Lena, who had already been sent reeling by Eve Teschmacher's betrayal earlier in the season. The cruel irony is that Kara had wanted to tell Lena the truth throughout season 4, but had been convinced that it was better to wait until the situation with Lex had been resolved. While Lena showed up to their weekly game night smiling, the penultimate scene of the Supergirl season 4 finale showed just how hurt Lena was, day-drinking at work and breaking the glass in a framed picture of herself and Kara as she put her drink down.
Leviathan Is Coming
One of the final scenes of the Supergirl season 4 finale started out as a seeming non-sequitur, with an elderly woman sitting down next to a younger woman at a bus station. After making some polite small talk, the older woman says that the younger woman will never be able to hide, no matter where she goes or how many times she changes wigs. The younger woman reveals herself to be Eve Teschmacher, who begs the old woman to let her go, saying that she did everything that was asked of her in regards to Lex Luthor and pushing him to develop a new source of energy that would disrupt society on a global scale. The old woman says Eve's freedom isn't part of the agenda, as Eve realizes that everyone else on the block is discretely watching her. The scene ends with the old woman spookily saying "Leviathan is everywhere. Leviathan is everyone. And Leviathan is coming."
Fans of DC Comics will recognize the name of Leviathan as that of a criminal syndicate that was created by Talia Al Ghul as a rival to the League of Assassins. Given that the organization is known to have the capacity to create their own metahuman agents and brainwash nearly anyone into working for them, it's a safe bet that they will be the chief enemy facing Supergirl and her friends next year. Interestingly enough, Leviathan is also currently being set up as the chief menace in the monthly Superman comic book. However, it is unclear at this time what characters from the comics we can expect to join the cast of Supergirl in season 5.
J'onn's Brother Is Back, Thanks To The Monitor
The Monitor, last seen in the Arrow season 7 finale, makes two appearances in the Supergirl season 4 finale. In the first, he appears in National City as a figured in a black hooded robe emerges from a breach, confused and disoriented. The Monitor explains to the figure that he freed him and brought him to Earth, so that he could avenge himself upon the brother who wronged him. The Monitor departs as the figure lowers his hood to reveal himself as a Green Martian, who swears that he will have his revenge on J'onn J'onzz.
While not identified by name in the episode, the credits confirm what comic readers immediately suspected - the black-robbed Martian is Malefic J'onzz, brother and arch-enemy of the Martian Manhunter. In the comics, Malefic was a criminal who was stripped of his telepathic powers and went on to engineer a telepathically-spread plague that killed all of the Green Martian race except for J'onn. The history of Mars is drastically different in the reality of Supergirl, however, so comic fans can expect to see an entirely new reason for the animosity between the two brothers.
How Supergirl Sets Up Crisis On Infinite Earths
The final scene of Supergirl's season 4 finale shows the Monitor manifesting in Lex Luthor's bunker, where Lena apparently left her brother's corpse to rot. The episode ends with Monitor regarding the body and lifting his hand, which begins to glow with a blue energy not unlike that of the breaches we have seen him form in the past. It seems likely that the Monitor may be restoring Lex Luthor to life, but we may have to wait until Crisis on Infinite Earths to learn why.