The two principal characters of this story are Kim Min-kyu (Yoo Seung-ho) and Jo Ji-ah (Chae Soo-bin). Kim Min-kyu lives a luxurious but isolated life due to a severe allergy to physical contact with other human beings. He develops extreme life-threatening rashes that rapidly spread over his body whenever he makes any form of skin contact. Jo Ji-ah is trying to make it in life by creating businesses based on her inventions, notably two networked heart-shaped lamps. The two first meet when Kim Min-kyu hires her to purchase an exclusive toy, but the transaction leads them into a fight.
Kim Min-kyu is the chairman and largest shareholder the KM Financial company. He inherited his shares from his parents, who died in a car accident 15 years earlier. He was brought up by his father's friend and colleague, Hwang Do-won (Son Byong-ho), whose son, Hwang Yeo-chul (Kang Ki-young), is the same age as Kim Min-kyu. Both are executives at KM Financial and Hwang Yeo-chul has become the CEO.
KM Financial owns the Santa Maria team. The team is headed by professor Hong Baek-gyun (Um Ki-joon). Professor Hong's team has secretly created an advanced humanoid robot called AG3 (pronounced Aji 3), whose face and other features he modeled on those of his ex-girlfriend Jo Ji-ah (the sound of the robot’s name is Ji-a [AJI 3] backwards).
Hwang Yeo-chul and his father Hwang Do-won try to sell the Santa Maria team to Bold Group, owned by a foreign investor, Martin, who has learned about the Aji 3 and wants to use it as a weapon. To convince Kim Min-kyu not to sell the Santa Maria team, Hong Baek-gyun sends the robot to him as a demonstration. Just before the Aji 3 is delivered, a crucial internal mechanism inside the robot's body is accidentally damaged by a single drop of water. Hong Baek-gyun approaches Jo Ji-ah, asking her to pose as the robot since she was its original model, giving the team time to repair Aji 3. Desperate for the 10 million won fee, Jo Ji-ah agrees, and impersonates the robot under the supervision of the Santa Maria team and the robot's computer brain.
Gradually, Kim Min-kyu falls in love with Jo Ji-ah, believing that she is Aji 3. He is disturbed by this, and reboots the Aji 3, erasing its memory. Professor Hong Baek-gyun replaces Jo Ji-ah with the real Aji 3. Jo Ji-Ah, who has also fallen in love with Kim Min-kyu, leaves the city to work in her aunt's restaurant, in an attempt to forget him.
Kim Min-kyu discovers that Jo Ji-ah had replaced the robot, and has a sudden and violent allergic reaction, which nearly kills him. Hong Baek-gyun saves him by injecting him with ephedrine, but he is furious and breaks completely with Jo Ji-ah. Nevertheless, in a climactic scene, Jo Ji-ah confesses everything to Kim Min-kyu and they rediscover their love, this time between humans.
Before the reboot, Aji 3 had stored all the video footage and data from the trials with Kim Min-kyu in a secret cache. This cache gets hacked, revealing everything about Kim Min-kyu's illness. This knowledge is used to attempt a take-over of KM Financial, to remove Kim Min-kyu as chairman of the company and complete the sale of the Santa Maria team and Aji 3 to Martin. Kim Min-kyu manages to thwart the attempt at the last moment. He uncovers evidence that Hwang Do-won and Martin were involved in the murder of his parents, and restores the Aji 3 to the Santa Maria team. The Santa Maria team successfully launches a new robot after changing its appearance.
Pretty intense