A lot was obviously revealed in the six minute extended trailer (watch below) with the pilot expanding upon the footage.
Kara's leaves Krypton at the same time as Kal-El, with her mission being to protect her baby cousin. Her pod is directly connected to Kal-El's and the coordinates to Earth; however, as Krypton explodes, Kara's pod is sent to the Phantom Zone.
Kara than wakes up Earth not knowing how she got there. Her cousin (never referred to as "Superman" in the episode) finds her and takes her to the Danvers (Dean Cain and Helen Slater play her adoptive Earth parents).
Kara works for Cat Grant (Calista Flockhart) at Catco in National City where she is Cat's assistant. Winslow 'Winn' Schott (Jeremy Jordan) and James Olsen (Mehcad Brooks) are introduced. It's noted "The Big Guy" still refers to James as "Jimmy Olsen." Schott, aka Toyman in the DC Comics, is the tech guy and has a crush on Kara.
Alex Danvers (Chyler Leigh) is Kara's Earth sister. She's off to Geneva when her plane suffers a malfunction - that's when Kara reveals her powers for the first time. Alex isn't happy that Kara used her powers and tells her not to do it again.
Kara needs to tell someone - to share in her good feelings - so she reveals herself to Winn on the rooftop.
The villain of the episode is introduced, which is Vartox (Owain Yeoman). He is responsible for sabotaging the plane and tells another alien, The Commander (Faran Tahir), that a girl stopped the explosion. Vartox puts it together that since Jor-El saved his son, Alura saved her own daughter. It's then mentioned they have been hiding for too long, and that the "General's arrival is imminent." Vartox mission is to eliminate the human operatives, and now Kara has been added to the list.
Kara gets her Supergirl suit and goes on a few missions to save the day. A fire breaks out on top of a building, and when she investigates, she gets shot down by green-colored darts. Kara wakes up on a gurney where Hank Henshaw is introduced (David Harewood). Henshaw is the director of the DEO - Department Of Extranormal Operations. Her sister Alex is an Agent. The green darts were kryptonite. The DEO protects Earth from aliens and alien invasions.
Henshaw explains when Kara's pod crashed, something else followed her from the Phantom Zone - Fort Rozz, a Kryptonian maximum security prison for aliens, and that the alien convicts have escaped on Earth and are the most dangerous in the galaxy. Henshaw is reluctant to have Kara join them, and off Kara goes.
Back at Catco, James Olsen tells Kara he knows about her, that her cousin sent him to watch out for her, that the first thing her cousin did was save a plane, too. Kara then gets a message from Vartox that only she can hear. Supergirl heads to a power plant on the outside of town where the two battle. Vartox wields some sort of axe made of strange metal (possibly Nth metal?) that can cut Supergirl. The DEO intervenes, and Vartox jumps/almost flies away. Back at the DEO base, they take a piece of metal out of Kara's shoulder and analyze it.
Back at her apartment, Kara has words with her sister about Alex being an agent of the DEO. Some drama ensues with Alex eventually giving her a container from her pod ship which contains a hologram of her mother, Alura. Alex says to suit up and back to the DEO they go. They convince Henshaw to let Kara help, and then they locate Vartox through use of the axe metal shard energy signature.
Supergirl flies off and battles Vartox. Alex was able to analyze the metal: If it gets superheated, it will explode. Supergirl uses her heat vision, destroying it, which explodes causing Vartox to lose the battle. Vartox takes a piece of the metal and kills himself, but not before telling Kara that she has no idea what's coming.
Next, Kara talks with James Olsen on the roof where he gives Kara her cousin's cape. Her cousin is really proud and wanted her to make the decision to be the hero herself, like he did.
Finally - the main Supergirl series villain stands revealed.
Some sort of ship/control room is shown with a woman in black. She's the General and notes Kara is her niece! It's Alura's sister! Actually, it's Alura's twin sister as she, too, is played by Laura Benanti! Alura's sister mentions how she will rule Earth just like how she was supposed to rule Krypton. No one will stand in her way. Supergirl is as much of a threat as her cousin, because she is like Alura, etc.
Alura's sister then orders Supergirl to be killed, and that no one will stand in her way.
The Vartox/aliens want/wanted revenge on Supergirl because Alura was the judge who sent them to the Fort Rozz prison and Phantom Zone.
Winn mentions "Super Friends."
Kara's cousin is referred to as the "Man of Steel."
Alex is a bioengineer and was recruited to the DEO because she lived with Supergirl.
Supergirl has super-hearing, x-ray vision, susceptible to kryptnonite, and can't see through lead.
It's possible Alura's twin sister might be a version of Ursa from the comics?
(hat tip to "Superman is not human" for sending in the info)
Supergirl extended trailer: