Quiz(2000)

A woman's young son is kidnapped and the only clue to his disappearance is a note starting with the words "This is a quiz." What follows is a fast-paced and stylish whodunnit which leads chief investigator Kaoru Kiriko of the Special Investigation Team down a trail of increasingly perplexing riddles that all the while seem to mock the efforts of the police. Street-smart Kaoru may be the kidnapper's intellectual match, but as she gets closer to her mark, she finds herself becoming the hunted rather than the hunter.

(Source: TBS)