It is estimated that there are still around 200,000 ninjas in Japan today. These ninjas work for the secret organization NIN (National Intelligence of NINJA), carrying out deadly missions such as assassinations and sabotage. However, there is another organization that opposes the NIN: UNDER NINJA. Known as UN, these ninjas oppose NIN at every opportunity.

One of the most important members of Under Ninja is a high school student named Kuro. Kuro is a loner at school and one day, he is sent to the front line by the UNDER NINJA. Here, Kuro learns the dark secrets of the NIN and becomes involved in the secret war between the two organizations.

Under Ninja has been serialized in Kodansha's Weekly Young Magazine since 2018. The series was also adapted as an anime television series by Tezuka Productions in 2023. 

Under Ninja is an action and comedy series. The series is about the secret war of ninjas in modern-day Japan. The series shows that ninjas continue to exist today as they did in the past and that this war is still going on.

The series has received positive reviews both as manga and anime. The series has been acclaimed for its action sequences, comedic elements, and unique take on the place of ninjas in modern Japan.