As Ming gets ready to proceed with her plans for artificial insemination, her husband Chen, is stressed and fantasizing about street races and infidelity. The sperm donor, Fitz, their medical school classmate and Ming's former boyfriend, is considering reinstatement at the hospital which stirs up a lot of unresolved issues. His return would also create more tension between the trio since he wouldn't be an absentee donor after all, but working alongside the couple every day.

Fitz and Ming talk about their imminent insemination plans. Fitz tells her that since he's decided to stay in Toronto and work at Mercy, he'll understand if she changes her mind. But she still wants to go forward even though she knows it will be uncomfortable for everyone. Their plans incite the doctors to reflect back on their early days as residents in the hospital and and the professional and romantic rivalries that took root.

The present. Jealous and sensing ulterior motives in Fitz's return to Toronto, Chen confronts him with a stash of love letters he wrote to Ming. The two come to blows then reflect back on a time during their residency when they helped each other out during a Code Blue. At the ensuring peer assessment review, Fitz takes stock of his mettle as a doctor, his motives as a friend and his true feelings for Ming.

It's Christmas Eve, and chaos reigns in the emergency room. Fitz contends with some hard-assed cops and a belligerent patient. It's a contentious situation that he initially handles well, but as the evening progresses he spirals out of control to a very self-destructive place. When Chen finds himself dealing with a family of a man who died in a brothel, he starts to question his own views on medicine and family. Meanwhile, though she's not embroiled in the turmoil of the hospital, Ming is confronted by her father's strange solution to her fertility problems.

Fitz travels to Guatemala to rescue a man who has bleeding in the brain. However, because the bleeding is so severe, the man will not make it back to Toronto due to the cabin pressure. Meanwhile, Ming has to perform a emergency delivery when her patient's baby has its neck trapped in the umbilical cord. And Chen finally realizes that there is more to life than medicine.

After Fitz takes a sick patient from China to Toronto who dies later, he is diagnosed with a mysterious respiratory illness. He is immediately put into isolation, but has infected Chen who examined him. Ming is in quarantine at home where she starts to rethink again about adoption. Bearing the scars of never knowing her father, she doesn't know if she wants to make a child go through that situation again.

Ming finally sees her dream of being a mother come true, though the donor plans she and Chen made with Fitz undergoes some changes. Through this, Fitz learns how to love and be loved, and Chen feels confident to pursue a writing career.