Season 2 of "Doctor Foster" opens with gemma arriving at the hospital for work while texting Tom to get her ground coffee. Gemma has now become the highest-ranking practitioner at her workplace. Upon her arrival, she greets Ros, who is now engaged and shows Gemma her ring. Simultaneously, a set of red envelopes are distributed to houses in the neighborhood, including Gemma's. Tom finally opens one and learns that it's from Simon and Kate, inviting him to their wedding and housewarming party as they return to town after living in London for a few years. Tom is shocked, but Gemma calms him down, telling him it's okay. However, she herself feels uneasy that night because of this news. She checks on Tom, who tells her he doesn't want to go to his dad and his new family's celebration. Gemma is grateful for that and wishes him goodnight. Later, she receives a text from Simon repeating his invitation to ensure she and Tom will attend. The next morning, she drives past their street and decides to turn around to check out Simon and Kate's new house. It's a giant and modern house, much like one of Kate's parents', with an indoor pool. Gemma sees a door open, enters, and walks around the house. At that moment, Simon arrives and sees her through a glass window. Simon tells Gemma that he's now in a better state in life, which is why he can afford this huge house, not because he hid money from her during the divorce. He suddenly mentions that he thinks about and even dreams of Gemma sometimes. He then asks if Tom is coming to his wedding, and Gemma tells Simon that Tom has another party by the weekend. Simon presses for Tom to come, citing family first, but Gemma says it's up to him to decide whether or not to come. Simon criticizes Gemma for not completely moving on and for still being difficult with him and Kate. He tells her she still lives in her old ways and sneaked into their new house while he has already moved on and is enjoying his new life. Gemma is offended and leaves. She thought she could live her peaceful life as a single mom in this little town, but no, they had to come back and stir things up again. At work, Gemma talks to Ros about Simon's wedding, and Ros is already aware. Ros then tells Gemma to try dating apps because they worked for her. Gemma checks up on a patient, and he asks her out for lunch. Gemma declines, saying she's not looking to date anyone right now. During a break, Gemma discusses with Ros again about Simon and wonders how he can afford the fancy house in the posh area of town when just two years ago, he was bankrupt. Ros reminds Gemma that it's not their business, though. Gemma returns home and Anna shows her that she also received Simon's invitation, which Anna tears up to demonstrate she's not going to attend. The day arrives, and Gemma encourages Tom to go to Simon's wedding with a gift for Kate. Anna helps her drop him off at the party while Neil walks Gemma to her cab. For some reason, Anna is now comfortable with her husband and Gemma around each other. Gemma ends up going on a date with James, the patient she previously rejected. James is eager to talk to Gemma but she seems distracted and keeps checking her phone. James notices this and quite upset but still tries to crack a few jokes. At that moment, simon sends gemma a picture of him with Tom holding a glass of wine. Gemma loses it and excuses herself to leave, but James doesn’t want to end the date like this and asks if he can accompany her. They go to the party, and Chris almost doesn't let them in. Gemma finds that Ros and her fiancé are already there, even though just a few days ago, she told Gemma that she wouldn’t go.

Simon soon comes to greet Gemma and James, then pulls Gemma aside to talk. Gemma expresses her discontent about her 15-year-old son getting drunk, but Simon says he just had one glass of wine. He thinks she's just looking for a reason to make a scene and asks her to leave but she refuses. Gemma touches simon’s shoulder and make sure kate who is watching them from afar see this. Tom see gemma and realises that her date James is his teacher. James asks gemma if they should leave, but gemma doesn’t want to stay quietly at home knowing her friends are celebrating someone who ruined her life. gemma wants to stay and make everyone remember what they did to her but james can go if he wants too, however james decides to stay. Its time for a toast, and simon and kate give speeches. kate apologize for their troublesome beginnings but looks forward to the future and a new beginning. Then she looks at Gemma, thanking her for coming and expressing hope that she wishes them well, especially for the kids. Gemma is silent for a while and then awkwardly says yes. Kate continues to thank her family, while Gemma wanders the house on her own. She goes upstairs and finds the room they made for Tom. The gift for Kate is on the floor. Then she finds Simon and Kate's master bedroom, gemma opens the nightstand drawer, and casually checks out the vibrators and lube inside. Simon walks in and again asks her what she is trying to do there. Gemma says that after what he did to her, he needs to face some karma and she wants to see that. But it seems like his life is going so well, and it isn't fair. However, Gemma noticed something when she touched his shoulder earlier; she felt a reaction from him that made her realize he still wants her. Gemma's self-esteem is destroyed after Simon left her for someone younger, and now she's ecstatic to hear Simon admit that he's still attracted to her. Gemma walks out and bumps into Kate, who is walking upstairs. Gemma tells Kate she was with Simon in the bedroom, making Kate angry. Gemma then hands her the present and tells her not to show it to Simon.

Gemma tries to find Tom but can't find him anywhere and starts to worry. Finally, she finds Tom looking lost and drunk, and they get in the cab with James. When they get home, Tom calls Gemma desperate for going on a date with his teacher. He apologizes afterward and leaves Gemma alone in the kitchen.

The next day, Gemma goes to work and is lectured again by her obnoxious new colleague, who asks her to stay professional no matter what is going on in her life. Gemma finally finishes this exhausting meeting and goes out to meet her patient. The assistant informs her that someone sent her flowers this morning. Gemma is shy but happy, thinking they're from James. While calling him to say thanks, she finds a card that reads "bitch" and immediately hangs up the phone.

Gemma asks Gordon for one minute and runs into her office to call Tom while sobbing. She tells him that today is difficult and his dad isn't fair. After she hangs up, Gemma leaves the hospital and drives to Simon and Kate's house.

Kate opens the door and tells Gemma Simon isn't home. Gemma shows her the card and accuses Simon of playing with her emotions. When Kate doesn't believe her, Gemma tells her that last night, Simon told her he still finds Gemma attractive. Kate laughs it off and tells her that no one hates her, but they just don't think about her at all, and threatens to call the police if Gemma shows up at their house again. Gemma comes home and finds Simon there waiting for her. Here, he reveals his true intention: to force Gemma out of his hometown and take Tom with him. Simon gaslights her, claiming that his cheating was insignificant and that she ruined the family just as he did. he insists that Gemma needs to accept the situation and move on so that everyone including herself can be happy. At that moment, Tom comes back home and grabs a bag to move to Simon's house. Gemma, shaken, tries to ask him what is happening. Tom coldly silences her and asks his dad to leave with him. Gemma runs after him, trying to stop him from leaving with Simon, but he ignore her. Anna comes out of her house and tries to talk to Gemma, but Gemma ignores her and goes to get a small box from her drawer. In the evening, while Tom is having dinner with his new family, Gemma uses chemical to destroy her wedding ring.