In the break between the holidays, the criminal underworld loaded up on its getaway trucks, ski masks and added a couple of rockets just in case The Girl of Steel showed up again to foil their plans. Sure enough, there she was, swooping in, heat vision a-blazing, to stop the van. Her clean-up could have used some work as two of the thieves ran away, only to get a throttling from Guardian. He too needs some help on his finishing moves because one escapes and almost kills Wynn before Guardian puts him down, leaving Wynn a quivering mess. What a way to start Season 2.5!
Kara herself isn't too excited about it though. In her mind the real important stuff is helping people and not fighting crime so it's a good thing that the mother of a runaway teen shows up at CatCo looking for her missing daughter. In fact, even Mon-El is feeling a bit restless since the "Incident" (which is apparently just how people in Superhero universes describe alien invasions) because he's out bartending and tells Kara she should mind her business. A tip from Maggie Sawyer about a rash of missing people in National City leads us to a scene of a young man who thinks he's finalizing the last bits for a medical study being whisked away, with Roulette, last season's new Villain appearing and taking credit for the capture.
Kara's new information brings them to the National City Clinical Center, a plain looking doctor's office, where the same creepy doctor who'd previously kidnapped Roulette's last victim, greets her and Mon-El, excited for more specimens. When he was ready to send them through his finally-revealed-interplanetary-transportation-device, their powers quickly dismissed his guards. He's then revealed to be a reptilian alien. Kara dives in after him through the device, telling Mon-El to call the DEO.
Aliens never listen, particularly to each other. Kara, on the other side of the portal after Mon-El tells her not to go, finds herself in combat with two more aliens and her hands bleeding after a series of well-placed knockout punches. Looking up, she discovers that wherever she is has a red sun. A quick note about the Superman mythos for the uninformed, Kryptonians (and Daxamites) get their powers on Earth because of its yellow sun. Under a red sun, like the one that Krypton and Daxam orbited, both races lose whatever powers they gain under different color stars. The effect is usually gradual as the yellow sun's radiation is drained from the body but for the sake of one hour, network story telling it's immediate (we'll let it slide).
She's followed immediately by Mon-El, who also ignored her order to go tell the DEO what was going on and where they'd gone. They're quickly captured by a strange alien named Jo who tells them they are on the planet Maaldoria, the hub of the intergalactic slave trade. Kara has one goal, to save the hostages and her only plan is, let's get captured! Once in custody, Supergirl confronts Roulette and her new ally, a Dominator, and tries to prevent the hostages from being shipped off throughout the galaxy. She even takes a few minutes to chastise Mon-El, who is still trying to convince her that her constant heroics are only going to lead her into more trouble. Several stun blasts later, the hostages' revolt around a powerless Supergirl who'd defied their captors, overpower them and make their escape attempt.
Meanwhile on Earth, the missing Kara is tracked to the clinic where the "Stargate" is found (Wynn's nerd cred grows). J'onn J'onzz can't go rescue the pair because the planet's atmosphere is toxic to Martians (D'OH!!). So it's up to the dynamic duo of Alex Danvers and Wynn (?) to lead the rescue. Wynn is still shook to the core after his near death experience with Guardian but Alex tells him it's time to put on his big boy pants and go rescue Kara.
Fleeing hostages and the rescue squad meet up in the alien citadel while Wynn (marveling about being in space, chanting over and over, "I am not a red shirt!") has his own moment of redemption, defeating an alien that's discovered him at the portal. In the midst of the escape, a Maaldorian has Mon-El in his sights but is prevented from harming him by the Dominator. A connection between the two races has yet to be established but the precedent exists in the comic book story "Invasion" that the four part CW series Crossover was loosely based upon.
It's just not Supergirl without a last minute rescue and when the aliens appear in one of their ships and the girl that Kara was looking for initially winds up in the clutches of a Maaldorian, it becomes rescue time. This time it's Alex who saves the day with a YELLOW SUN GRENADE (yes, you read that.)! Kara is instantly restored to full Kryptonian splendor and a blast of heat vision destroys the ship and sends the last alien running.