When Min and Yu Na's parents go missing, leaving behind only a note, the kids must make it through the day without anyone finding out - including a suspicious stranger who tries to follow them home.

Min and Yu Na learn of an important spy mission that their parents were meant to complete, involving a visit to a local enemy agent's house. With their friends' help, they embark on the mission themselves.

On Parent Teacher Night, Min and Yu Na enlist Mr Potts to step in as their grandfather. Things go well until a strange voice intercepts their radios and they realise there's an enemy agent present.

To reveal the bad spy, the kids invite the three suspects from Parent Teacher Night over for a day of interrogation, using some truth serum stolen from their parents' spy room. The answers surprise them.

The kids organise an excursion to the Botanic Gardens to intercept Mr Turei's mission - but their plan is derailed when their Halmeoni shows up unexpectedly, distracting them from their goal.