After J’onn broke Supergirl’s kryptonite-induced fall, he brought her to the DEO where her prognosis wasn't good at all.
Everyone rallied together to determine how to save her before it was too late.
However, the episode veered from the fight for Supergirl's life to provide us with some context surrounding the Agent of Liberty's backstory.
Before he was the voice for the anti-alien movement, Ben Lockwood was just a rational and open-minded professor at a local college who tried to teach his son not to fear or despise the aliens like his father, who referred to them as "roaches," had.
But Lockwood's acceptance started running thin when catastrophes began to take everything he ever cared about -- his home, his job, his family -- away from him, and he eventually became radicalized.
Preaching his prejudice to the class cost him his job, and he snapped after the death of his father.
Ben, along with some other steelworkers, burnt down the Nth metal factory and began killing aliens in town.
Mercy Graves got wind of his killing spree and decided to proposition him.
She told him that they could spearhead a much larger movement, gave him an armored suit which he completed with a steel mask from his father's factory, and they began planning to expose the President and kill Supergirl.
In the present day, Brainy admits that Supergirl's system is too weak to completely filter her blood from the kryptonite.
Alex calls in Lina as a reinforcement who brings her exoskeletal suit. She says it will keep Supergirl isolated and allow her immune system to detoxify properly.