Sachiko Tachibana interviews for a babysitting job at the home of famous pianist Rena Hanai, who is somewhat surprised by Sachiko's persistence.

Sachiko decides to sneak into the room of Rena's father Minoru, a professor of criminal sociology, to search for evidence of Rena's crimes.

Determined to make Rena suffer, Sachiko sets her sights on baby Kokoro. A covertly recorded conversation between Rena and her manager leaks online.

After seeing how Rena truly feels about Kokoro, Sachiko rethinks her tactics and soon realises that the key to revenge may lie in Makoto instead.

Under the pretense of preparing for Rena's surprise party, Sachiko arranges to go shopping with Makoto. However, Rena soon grows suspicious.

Sachiko learns from Minoru that Makoto's mother was murdered. Later, Makoto feels something familiar when he hears Sachiko playing the piano.