My Son's Ex-Girlfriend Became My Subordinate

Koji Takase is a single father and section manager at a major corporation, living an ordinary life despite being obliviously popular with female coworkers. When new employee Himari Aizawa joins his department, she claims to know him from the past. During her welcome party, she suddenly kisses him and asks, "Do you remember me now?" What secret connects Himari to Takase?