This is a summary based on our understanding of the series.
Please also note that this summary contains spoilers.
Daegu’s Venus or Kang Joo-eun used to be very pretty during her teenage years. When she became a lawyer, she became very fat. Her very fat friend, Oh Soo-jin, became very thin and ended up stealing her boyfriend of 15 years.
She met Kim Young-ho on a trip when she fainted. He turned out to be John Kim, a well known trainer in the states. He ended up being her coach along with his other friends. The two of them later on fell in love (and ended up) with each other.