A famous fortune teller who claims to be spiritually linked to Marie Antoinette and calls herself 'Madame Antoine' goes head-to-head with a cold psychotherapist who runs a clinic also by the name 'Madame Antoine' and aims to prove that true love doesn't exist.
The foremost authority in women's psychology, psychologist SOO. Is he ousted from his domain by the intuitive Helga, who sees through everything at a glance? A confrontation unfolds around Madame Antoinette's name! And the first client, gymnast Mari, has arrived!
As Helga and Soo begin their counseling sessions, a psychological experiment on love unfolds simultaneously. Between counseling and psychology, a game of push and pull ensues between Soo and Helga. Meanwhile, during gymnast Mari's session, Soo inadvertently hurts her, leading the session over to Helga...
While secretly preparing an event at a hotel for Helga, Soo is caught off by her sudden appearance. Flustered, he confesses his feelings to her. Meanwhile, top star Ju-ni suffers a seizure during rehearsal and seeks help at the center. Using Ju-ni's counseling as an excuse, Helga and Soo starts their first date.
To win Helga's favor, Soo buys her a luxury bag, but Helga playfully teases him in return. Meanwhile, Seungchan and Jiho join forces in their mission to charm Helga, while Ju-ni's gets furious by jealousy towards Helga.
After a date, Soo forgets his role as Experiment Man A with Helga, caught between his duties and growing romantic feelings. Meanwhile, Helga finds herself in a tense situation with Yurim's colleague, Seongho, leading to get detained by the police.
Soo lies to deceive Helga about the experiment, yet he wrestles with guilt over her. Worried that his personal feelings might disrupt the experiment, Soo quits the Experiment Man A and recruits a new replacement. Meanwhile, Helga finds herself increasingly intrigued by the new Experiment Man A.
Helga ends things with Soo after he lies about his relationship with the professor. Determined to win Helga back, Soo devises a new strategy. They both encounter Yuseon, a famous magician suffering from dissociative identity disorder during a magic show, and bond while counseling Yuseon together. Meanwhile, Seungchan expresses his love for Helga by painting on a wall.
Soo and Helga confirm their love with a passionate kiss and continue their secret relationship. However, Helga's lingering suspicions about the experiment lead her to ask Soo to delete experiment-related documents, which deeply troubles him once more. Meanwhile, Helga organizes an event to comfort Soo, who is grappling with childhood trauma, but Soo finds Helga's efforts somewhat awkward.
With Soo's heartfelt confession, he and Helga become perfect lovers. However, witnessing Helga being used in Soo's experiment, Seungchan cannot stand idly by. He attempts to reveal the truth to Helga, along with Jiho. Meanwhile, while Soo and Helga are on a one-day trip, Helga discovers the code to Soo's hidden safe and tries to open it.
Finally discovering the truth of the experiment, Helga assigns Seungchan and Jiho to monitor Soo's every move, plotting her revenge. Soo, sensing Helga's coldness, tries everything to thaw her heart, but Helga plans a counter-experiment to dismantle Soo once and for all.
As Helga begins a counter-experiment, she constantly picks fights and accuses him of losing interest. She demands that Soo undergo an fMRI scan to prove his feelings. As her second strategy, she introduces a young and attractive experiment woman to Soo. However, as Soo and the experiment woman grow closer, Helga finds herself extremely jealous.
Disappointed with Soo for continuing the experiment, Helga breaks up with him, leaving Soo hurt and confused by her sudden change. Despite her anger, Helga can't let go of her love for Soo. Meanwhile, Soo discovers that Tae-hwa impersonates him. While following Tae-hwa, Soo finally uncovers his secret.
Helga's jealousy erupts with the sudden appearance of Soo's ex-lover. At a party, Miran's confession of her feelings for Seungchan is revealed, causing her to flee in embarrassment. Meanwhile, Soo sends Helga a flower basket as a task. It is only upon receiving Helga's heartfelt video message that Soo finally understands the depth of her love.
Upon hearing about Miran's thyroid cancer, Seungchan goes to Jeju Island to find her. Meanwhile, Soo, realizing his love for Helga, decides to finally end the experiment and contemplates how to convey his true feelings to her. At the same time, Helga plans an unforgettable "special" date for Soo.
After Helga leaves Seoul, Soo struggles with the pain of her absence. He musters the courage to meet Yeonhee, but she barely remembers that she had abandoned him. Soo finds himself utterly broken and alone.
Helga is shocked by Yurim's pregnancy news, and Miran decides to undergo surgery after Seungchan's persuasion. To comfort Soo, Helga takes him to a place from his childhood, where he confronts his younger self. Meanwhile, Helga's anxiety grows due to Soo's unpredictable behavior, and she receives a mysterious letter.
