Asher tells Will she's leaving early for a meeting and won't let him drive her. (We'll see what she's doing, especially since she forgot her phone.)
Marcel treats a little boy who has a blood buildup in his lungs. Afterward, Nat asks Marcel if he ever thought about having kids. Marcel says kids aren't his speed. Their conversation is interrupted by two detectives from New Orleans who want to talk to Marcel about a cold case.
Meanwhile April sees Ethan walk in. She follows him to give him his key back. He seems upset.
A 12-year-old kid is brought in. He was found in the library with a needle in his chest. The kid fights back and pushes the needle deeper in. He is sedated.
Anna comes in with Dr. Charles. She wants to volunteer. Before she can be sent to pediatrics, she overhears Lanick tell Charles about the kid having four sewing needles in him.
Will sees a patient with cellulitis who he has seen before. The patient doesn't want him to call his wife.
Billy needs surgery. After comforting Billy's mother, Nat asks Marcel about the detectives. He says he's a suspect in a murder from 7 years ago.
Charles talks to the needle guy. The patient is a PhD student who thinks something is wrong with his heart and puts needles in his chest to fix it. He needs a needle removed before it goes into his lungs but Latham is hesitant because he might re-insert it later. Charles convinces him to do the surgery if he gets consent. Meanwhile, Anna gets kicked out of Pediatrics for being on her phone and says everyone is uptight. Maggie takes her to the waiting room to help out there.
Will sees a patient who is clearly high on heroin and has an arm rash. Asher comes in and Christian flirts with her. Asher tells Will privately that Christian is a part of her past she is trying to leave behind. She says he called her after the meeting and she came over and brought him to the hospital. Will gives her her phone and says nothing about her obvious lie.
After getting an update from Marcel about Billy's surgery, Nat leaves the OR to find Goodwin informing the detectives they are not arresting Marcel in the middle of surgery. Goodwin makes the cops wait in the waiting room and tells Earl the security guard to notify her if anything happens. Billy's mother is nearby. Nat asks Goodwin if she thinks Marcel is guilty. Goodwin doesn't know.
Ethan rides along with some EMTs to a hostage situation. They find out that a nine-year-old kid was shot inside the store.Ethan stupidly runs inside to help the kid and gets held hostage too.
The cops are on the way. Gunman makes Ethan help him block the entrance with a refrigerator. Ethan convinces him to let him look at the kid. The gunman says he only stole a car and didn't shoot the kid. Ethan tells him to let the kid go and keep him as a hostage instead. The gunman agrees.
The store owner is brought into the hospital. April wants to know where Ethan is.
Nat is bothering Atwater about what he's heard about Marcel's case.
Trent freaks out when Charles tells him he'll be in the hospital a few days. He starts coding.
Dr. Moody arrives. Billy's mom called. She's worried about Marcel's legal problems interfering with Billy's care. Nat talks to Moody who reluctantly tells her that 7 years ago Marcel's baby died of leukemia but he's no murderer.
It turns out that Will's patient has stolen someone's ID and no one knows who he is. Also he is having a severe allergic reaction to his medication.
April arrives. SWAT says Ethan violated a billion protocols. They're going in as soon as they have eyes on him.
Maggie says she needs the room if Trent isn't going to do the surgery. Charles catches Anna texting again. She says she'll die if she goes to the science museum with her class because the boy she liked laughed at her when she asked him out and is now holding hands on the bus with someone else. She says her life is ruined and runs off. Maggie tells Charles 13-year-olds' hearts are wired differently.
Charles gets an idea. He has Latham paged. It turns out Trent has a heart defect that acts up when he's anxious. Latham can fix it. Trent asks how long til he goes home. Charles says they will discuss that after surgery.
Will meets the real Roger. He gave his friend his insurance card because his friend had no insurance. Will yells at him that that was reckless and Brian could have died.
Nat researches DNA mix-ups. Doris comes along with some blood for Marcel's surgery. Nat gets an idea. She randomly asks Marcel if he donated bone marrow. He did, a long time ago.
Ethan talks the gunman down and the guy wants to turn himself in but SWAT breaks in just then. Arrest without incident and April meets EThan outside and hugs him and tells him how scared she was. Ethan has to go give a statement.
The cops attempt to arrest Marcel the second he leaves the OR. Nat stops them with info about how bone marrow donoees can inherit the donor's DNA in rare cases.
Charles talks to Anna about how he faked a leg injury to get out of gym when he was her age because he was slower than the other kids and that it's important to find a way to deal with things.
Will feels guilty about not catching the patient's lie. Maggie tells him it's good he gives people the benefit of the doubt.
Marcel wonders how Nat knew about the bone marrow. She says she knew he was a good guy and she knows about his daughter's death. He asks her to keep that to herself. She agrees.
WIll and Asher talk. She says that she ddn'tl ie. A mutual friend at the meeting gave her the phone to talk to Christian. Will says he believes her.