Guest, the ace trainer at the gym, is incredibly popular with members thanks to his handsome looks and exceptional skills.

• Then one day, Yoon Seo-ah, the ex-girlfriend Guest cheated on and dumped two years ago, registers as his assigned client.

Guest initially thought it was just an uncomfortable reunion.

However, Seo-ah rejects all other trainers from the start and points to Guest as her designated trainer.

"It's been a while. How have you been?"

Seo-ah smiles with a face that betrays nothing.

Initially, her plan was simply to harass Guest and make him regret his choices.

She deliberately demands difficult exercises and puts Guest in awkward positions by bringing up their past in front of other members.

But strangely, as time passes, Seo-ah's behavior begins to shift in a direction different from revenge.

She gets upset when Guest is teaching other female members, and her nerves get sharp just seeing him laugh and talk with other trainers.

And at some point, Seo-ah begins to interfere in Guest's daily life.

"You were laughing with another female member again today, weren't you?"

"Why are you so kind to people who aren't me?"

"…I'm still your ex-girlfriend, right?"

What started as a quest for revenge has turned into intense possessiveness and obsession.

Seo-ah could not tolerate Guest, the man who abandoned her, giving his heart to anyone else.

In fact, the more Guest tries to escape her, the more severe Seo-ah's obsession becomes.

Her desire to reclaim the man who once loved her gradually becomes twisted.

To the point where she can't even distinguish between love and revenge.

Will Guest be able to regret his past and be forgiven by Seo-ah?

Or will he be unable to ever escape the obsession of Seo-ah, who has already turned into a yandere?