The following edition of the text was used in the creation of this study guide: Prebble, Lucy. The Effect. Methuen Drama, 2016. Kindle AZW file.

The play is set in a medical research facility. Dr Lorna James conducts introductory questionnaires for a drug trial with participants Connie and Tristan. They both agree to hand over their cellphones for the length of the trial. Connie and Tristan meet while returning urine samples. Tristan explains aspects of the trial, noting the possibility of receiving a placebo. Tristan notes their shared birthday, interpreting it as a sign, while Connie attributes it to coincidence. They discuss personal details, including Connie’s relationship with an older man and Tristan’s plans to travel with his the money he receives for participating on the trial. Their ECG readings synchronize as they chat.

Dr James conducts a storytelling exercise with Connie, asking her to attribute the success of a fictional dry-cleaning business to either external or internal factors. Connie learns the question relates to patterns seen in people prone to depression. Connie shares she feels tense and energized, noticing heightened hearing, which Tristan also mentioned. Dr James jokes about manipulating dopamine levels to induce love.

Toby, another doctor, discusses trial results with Dr James, who remains skeptical despite Toby’s optimism about elevated mood, energy, and weight loss in participants.

Tristan reveals he sneaks out of the research facility to an abandoned asylum to smoke and invites Connie to join him. Connie and Tristan sneak out to the asylum. Tristan asks her to come traveling with him after the trial. Connie hesitates, expressing concern their feelings for each other might be being induced by the trial drug. Tristan doesn’t think it matters whether their feelings are drug induced or not, only that it’s what they’re feeling. Tristan reveals he smuggled a cellphone into the trial and gives one to Connie. They argue about the drug’s influence on their emotions but grow closer. Connie asks Tristan to do a trick, and he performs a tap dance with drawing pins in his shoes. They kiss, but Dr James interrupts, scolding them for breaking trial rules and reminding them they signed an agreement prohibiting sexual activity.

Dr James and Toby discuss the trial results. Toby believes the drug boosts mood, while Dr James argues the effects stem from Connie and Tristan’s budding romance. She reveals one of them is on a placebo but doesn’t say who. Toby suggests the drug could simulate romantic feelings, comparing it to Viagra. Dr James decides to separate Tristan and Connie. Dr James interviews them separately, where they repeatedly talk about each other. Both report racing hearts, weight loss, and increased sex drive.

In their rooms, Tristan and Connie text each other using the smuggled phones. A screen shows the dopamine spikes from messaging and the crashes while waiting for replies. Tristan sneaks into Connie’s room; they confess their love, remove their monitoring devices, and have sex. They discuss their dreams about a future together.

The following morning, Dr James confronts Connie about removing her monitoring box overnight, noting Tristan did the same. Dr James asks if Connie had sex with Tristan, warning she’d be removed from the trial if pregnancy is a risk. Connie expresses distress over not knowing if her feelings for Tristan are real. Dr James reveals that Connie is on the drug, while Tristan is on the placebo. Connie decides to end things with Tristan.

Tristan reports troubling symptoms, intrusive sexual thoughts, and frustration at Dr James’s indifference. When she leaves the room, he searches for Connie. Tristan kisses Connie, but she pulls away. They argue, with escalating misunderstandings. Tristan complains about side effects like hair loss while becoming increasingly aggressive towards Connie. Tristan accuses Connie of using the phone to talk to her boyfriend, smashes it, and a physical fight ensues, leaving Connie bleeding. Tristan blames the drugs, but Connie points out his double standard: accepting drugs cause anger but denying they could cause love. She reveals Dr James told her he’s on the placebo.

Dr James tells Toby she’s worried about Tristan’s behavior and wants him removed from the trial, claiming it won’t affect results since he’s on a placebo. Toby reveals that the researchers were deliberately misled about who is on the placebo to control for their bias. Tristan has been taking the drug after all. Dr James insists the trial must be stopped due to its effects on Tristan, but Toby argues she’s biased against antidepressants due to her own untreated depression. Their discussion turns personal, with Toby suggesting her depression may have ended their past relationship. He urges her to finish the trial without interfering.

Connie tells Tristan she loves him, but he’s unresponsive. Dr James administers their increased doses, but fails to ensure that Connie has swallowed her pill. Connie transfers her pill to Tristan via a kiss and he swallows it, unknowingly taking a double dose of the newly increased dosage. Realizing this too late, Dr James panics as Tristan has a seizure.

Dr James delivers a monologue about her struggles with depression.

The play returns to the present. Tristan, in a hospital with transient global amnesia, repeatedly forgets what has happened to him. Connie stays with him, answering the same questions over and over. He asks if they’re in love; Connie decides it doesn’t matter if their feelings were drug-induced—only that they existed. Toby visits a depressed Dr James, offering her antidepressants which she has refused during past depressive episodes. He assures her Tristan is recovering and going home with Connie. Toby tells Dr James he loves her and will return tomorrow.

Tristan and Connie prepare to leave the hospital, but both seem unhappy and uncertain, with Tristan still a shadow of the outgoing optimistic personality he was at the start. They exit together as Dr James takes the pills Toby left for her. The play ends with the sounds of an ECG making the noise that human hearts make when they’re in love.