The episode opens with Supergirl flying carefree through the sky, monologuing about how happy she is. She sees a drone following her and tears after it; it evades her surprisingly well considering she has super-speed, but ultimately she blasts it out of the sky.
She takes a part to the DEO, accusing Henshaw of following her, but he denies having any knowledge of it. Alex comes and says she doesn’t recognize the (very advanced) tech. Henshaw leaves with it, and Alex tells Kara she can trust him.
When Kara gets to work, Winn is stressed from covering her phone. Cat won the Siegel Prize for women in media, and the ceremony is in Metropolis tomorrow — but Cat doesn’t think she can go, because her mother won’t watch Cat’s son. Kara offers to watch Carter for her so that she can go.
Cat tells Kara that Carter is special — he is gifted, but shy, and he needs special attention.
At a restaurant, Lucy is talking to James about work. He wants to know what she’s really doing in town, and Lucy tells him that she missed him and doesn’t like how their relationship ended. James blows her off.
When Kara comes to see if he’s okay, he says she left him for her career. Before they can go too much more in-depth, a bomb goes off and Kara has to go save a skyscraper. She does, but while she’s using her breath to blow out the fire, a drone watches and studies her.
Henshaw is about to hand off the investigation to the FBI, but Alex has figured out that the drone is powered by proprietary tech from Maxwell Lord’s company — and the same technology was used in the bomb.
Kara and Alex talk about Kara’s relationship with James, telling her not to talk to him about his girlfriend troubles. She says that since James knows Kara’s secret, she wouldn’t have to hide around him.
Kara gets a call from Cat, who asks how Carter is, and she flies out of the building at top speed to his school. She introduces herself to him, and he’s incredibly shy.
At Lord Technology, Max tells Henshaw and Alex that the lab attacked was his as well — part of a shell company. They ask him to narrow down the potential suspect pool, and when Max is uncooperative, saying he doesn’t trust the government, Henshaw leaves Alex behind.
At the office, Carter is nonplussed with anything, until Supergirl comes on the news. He’s clearly infatuated with her, which entertains Kara. Winn says he knows her personally, which Kara shoots down.
In Cat’s office, James is having trouble running the building; he doesn’t understand how Cat keeps it all together. While she’s there, Lucy calls. He starts to vent, but Kara cuts him off.
At Lord Technology, Max is impressed with Alex, saying she’s wasting her talents with the government. Both of them think they can help the world in their respective roles, and Max is unimpressed with Supergirl.
He gets a call, and there’s a bomb in one of his labs. Max insists on trying to defuse the bomb, but when he does, the timer merely speeds up. Alex calls Kara, and Supergirl comes rushing in to fly the very powerful bomb far into the sky and throw it away — but it throws her into the ocean and knocks her out.
When she wakes up at the DEO (after briefly seeing Henshaw's red, glowing eyes but not really taking it seriously because she was semi-conscious), she realizes that she’s been gone for a really long time, leaving Carter alone with Winn. She goes to Noonan's, where she meets up with Lucy.
Henshaw says they found a shot of the bomber from security footage at Lord’s lab. His name is Ethan Knox, and he’s a recently-fired member of Max’s staff. Supergirl wants to help, but Alex tells her that she’s spreading herself too thin and needs to focus more.
At the office, Carter and Winn are playing video games on Cat’s monitor array. Kara leaves to get them lunch.
Lucy is feeling pretty bummed, and asks if James is seeing anyone, particularly Supergirl. Kara says no. Lucy says the reason she prioritized work over James is that he prioritized Superman over her.
Alex asks Lord to cancel the launch of his high-speed train today because Knox will target it. Max doesn’t trust the government because his parents — who were government scientists — died at work due to poor safety conditions. When Alex says she’s sorry, he says that’s the first apology the government has ever given him. She begs him not to reject their help, but he’s insistent.
At Cat’s office, Kara apologizes for cutting him off about Lucy earlier. She tells him what Lucy told him at Noonan’s. She says that if she ever really had romantic love, she would fight for it. James says he doesn’t want to open himself up to that kind of hurt with Lucy again yet, but Kara says that if he’s not really ready to move on, it won’t be fair to whoever is next. She leaves to check on press credentials for the train launch. After bringing Carter his lunch, she heads to the DEO in costume, offering to keep an eye on the train.
Later, Lucy comes to say good-bye to James. The two argue, ultimately agreeing that they want what’s best for one another. They kiss, and Lucy leaves.
As Supergirl follows the train, James sits in his office thinking about the Lucy situation. Winn brings a pizza in, and finds Carter is missing. When the news suggests that Supergirl could be on-hand for the train launch, he realizes that’s probably where Carter went.
At the train station, Carter is bumped along with the crowd toward the train.
At the airport, the police find a bomb. Supergirl is heading toward it as James calls Lucy in a panic and Max helps Carter onto the train to “find his parents.” Knox is on board with a bomb strapped to his chest, and Winn — who isn’t being allowed on the train — sees him before the train leaves the station. He calls Kara, since she’s the only one who can catch up with the train.
Kara tells the DEO that she trusts them to take care of the airport and chases after the train, which she knows has Carter on board.
When Supergirl comes in, Carter is impressed. Max denies it’s even possible that Knox is on board. Supergirl has Max and Carter walk the passengers to the back of the train, and heads to the front, where she uses her vision powers to see Knox.
At the airport, The DEO shows up at the same time James does. When they tell him Supergirl isn’t coming, he finds Lucy. When she asks why he ran toward the bomb, and he says to save her, the two hug.
At the airport, Alex says there’s no way for her to see inside the bomb to disarm it. Henshaw demands she leaves him alone, telling her it’s an order.
On board the train, Supergirl chases Knox to the very front car, where he reveals the explosives and says that it’s rigged to go off if he’s removed from the train. He says there’s no way out for him, and Kara tells him to think about his daughter.
At the airport, Henshaw’s eyes glow and he peels open the bomb with his bare hands. reaching inside, he feels around.
Outside, Henshaw brings the now-disarmed bomb out to a cheering crowd, telling Alex it was just a dud to draw them away from the train.
On the train, Supergirl pleads with Ethan not to hurt anyone. She says she knows his daughter is sick, and Ethan needs him. He says that what he’s doing is her only hope, and that no one was supposed to know it was him. When she pleads with him not to kill all of the people on the train, he says he won’t — because she will save them. He presses down on a plunger, telling Supergirl she has thirty seconds to do so. She heat-visions the coupling that holds the other train cars onto the engine, and watches Knox and the forward car speed away as she stops to slow the rest of the train enough to keep it from the blast. Out over the water, Knox and the engine both explode, taking out a length of the railroad bridge.
At the DEO forensics lab, Supergirl pops open the bomb canister while Alex is working on it. She sees a remote failsafe, which she says doesn’t make sense. Supergirl also doesn’t understand how the bombings would help a sick girl. It’s then that she realizes Carter was on the train.
At CatCo, Carter is excited about meeting Supergirl — she’s “so much prettier in person,” but Cat wants to know what else he noticed. When Kara comes in, Carter asks if she can watch him again since they had such a great time together, but Cat suggests it’s not likely. Kara apologizes to Cat when Carter leaves, and Cat is clearly upset but she just lectures Kara about how too many people burn out trying to have it all before they’re ready, and that you need to build to the kind of success she’s had. As Kara leaves the office, she sees James and Lucy making out. On the news, Kara sees Lord on the news. He’s saying that he has failsafes for everything…which gives Kara an idea. She asks Winn to hack into hospital records.
At Lord’s apartment, Kara stands outside on the balcony, telling Lord that she knows Knox’s daughter’s care is being paid for by Lord. She thinks he forced Knox to plant the bombs, and she knows that it was a ruse. He tells her she can’t prove anything, but that whoever was responsible might be curious about Supergirl herself — that she was being tested. He says that the most interesting thing is that she chose to save a small number of people at the train instead of far more at the airport — and that if he can find out who on that train was important to her, he’ll figure out who she is. She tells him that she’s watching him, and that this isn’t over, and speeds off. He says, “The fun is just beginning.”