Ep 15. Solitude

Kara is back in National City after the departure of her Aunt Astra. With her adopted sister, Alex, still feeling guilty for killing Astra years ago during her escape from Fort Rozz, Kara does her best to be there for her. Meanwhile, Cat Grant is experiencing a huge crisis with the failing rank of her company, leaving her with no choice but to fire one of her workers. Still wary of Winn's attraction to her, Kara tells him she wants to keep their relationship professional. But Winn is upset about James taking Lucy, resulting in a push and pull drama between the two friends.

Elsewhere, Kara stumbles upon a millennial genius named Siobhan Smythe who landed a job at CatCo as Cat Grant's new assistant. But not all is well within the company, and when Kara visits Cat Grant at her office, she discovers the walls have turned to ice and something is causing a disturbance in CatCo. The cause of this chaos is revealed to be a rogue AI program called "Indigo," who has the power to take down the entire internet and even cause the explosion of a nuclear missile.

Not wanting to leave National City defenseless against Indigo and her destructive ways, Kara seeks advice from her cousin, Superman, but is disappointed when he does not offer any help. She also asks James to hack into the Fortress of Solitude and upload the necessary protocol to defeat Indigo. However, James is not keen on going against Superman's orders, and his relationship with Lucy causes further friction. When Kara tries to confront Indigo, she is captured and taken to the Phantom Zone, where she meets a former prisoner who has been left there for centuries.

With Kara trapped and helplessly watching, Alex and J'onn try to take down the rogue enemy alone, but to no avail. Siobhan takes advantage of the chaos and starts to show her true demonic colors, but Kara manages to break free from the Phantom Zone and rally a team of allies in the fight against Indigo. The group also discovers the true identity of Siobhan, who has been plotting revenge against Kara and CatCo.

As the conflict reaches its epic conclusion, Supergirl and the team are forced to make difficult choices that will change the dynamic of National City forever. The episode ends with Kara and Alex on a much-needed break, James and Winn trying to patch up their friendship, and Cat Grant making a shocking decision that will determine the future of her company.