The male lead is Ji Chang-Wook (Suspicious Partner, Healer). Let's just let that sink in! It is so good to have him back on the screen after finishing his military service. He plays Ma Dong-Chan, a television Producer/Director. His goal in life is to make the world a better place through his programs. He agrees to do the frozen experiment because he believes a good producer should do the things no one else will do. When he wakes up, so much has changed. His family lost their fortune and had to open a restaurant. His father was dead. His sister is a drunk divorcee. And his older brother (Played by the same actor as his father before the experiment) has a child and has given up his dreams to be a concert pianist. On top of all that, his girlfriend is now 20 years older than him and does not make his heart flutter. And his irritating assistant has become the production chief at the television station.
He also feels like he should protect the other participant, Go Mi-Ran, since he talked her into doing the experiment in the first place. As he watches over her and takes care of her, he begins to fall in love. This only intensifes his need to find a way to return their body temperatures to normal, so they can be together.
Dong-Chan is a spontaneous risk-taker, but he is also a thoughful, overly responsible person. The way he takes care of his family and the people he loves makes him a very likeable character. He is confident, but he doesn't come off as arrogant.
The other experiment participant is Go Mi-Ran. Won Jin-Ah (Life, Just Between Lovers) is a refreshing change from the fragile leading lady. She does the experiment hoping to be a part of the cure for her autistic brother in the future. When she wakes, up her biggest regret is the time she missed with her family, especially her younger brother. Her two best friends were told that she moved to America suddenly, with no explanation. They have moved on with their adult lives and Mi-Ran has a hard time identifying with them the way she used to.
Mi-Ran is only twenty four when she gets frozen, so she is still very young. She hasn't graduated from college or been in love. She often feels cheated out of the experiences that everyone else had while she was sleeping. Dong-Chan gives her a job as an intern at the television station and she throws herself into it, getting her first taste at the opportunity to build a career. It doesn't take long working with her charming producer for her to fall for him.
We see Mi-Ran as an enthusiastic free-spirit. She approaches each new experience with a positive outlook and the feeling that she is trying to make up for lost time.
The girlfriend who spends twenty years waiting for Dong-Chan to come back to her is Na Ha-Young. Yoon Se-Ah (SKY Castle, Love of Eve) plays this complex character. She is still in love with Dong-Chan, even though he has been very clear that he doesn't feel the same. This makes her pitiable. But she also agreed to keep the information about the experiment a secret in exchange for a promotion, which makes me feel less sympathetic.