Mark Lenard is Nagata, a member of a Japanese guerilla team sent to Hawaii in advance of the Pearl Harbor Attack. Nagato escapes from a psyciatric hospital after 25+ years to complete his mission. Why was he in a psychiatric hostpital is never explained. Nagato uses Ninja techniques and tools in his attempt. Nobody on the Five-0 team recognizes shuriken throwing stars. Nagato is not aware of the passage of time. Nagato navigates around Oahu, Pearl Harbor, and Honolulu without error, despite being in a hospital since the 1940's. Nagato has a deceased wife, and a daughter he thinks is his wife, in Honolulu. A member of the guerilla team has been living in Honolulu and assimilated into the society. How and why is never explained.
There is just no redeeming this episode.