Do Ki is a young man who lived more honourably than anyone else as a special forces officer. But when his mother is murdered by a serial killer, he spends his days in despair. Feeling betrayed and broken, Do Ki meets Jang Sung Chul, who takes him on at his company, Rainbow Taxi. Far from being an ordinary taxi service, Rainbow Taxi is a secret organisation that avenges victims who have not been protected by the law. Do Ki becomes a renowned driver who carries out satisfying acts of revenge. Meanwhile, Ha Na is a prosecutor investigating Rainbow Taxi. She is furious at their illegal behaviour but, at the same time, feels a sense of admiration seeing them resolve cases that the law cannot reach. Caught between those who believe revenge represents true justice and those who pursue criminals, Do Ki is searching for the true meaning of justice.