Complex sentence: When Japanese master spy Takeo Yoshikawa arrived in Honolulu, Dr. Kuhn would flash coded messages from the attic of the Kuhn household--a system that went undetected until the end.
Split into simpler sentences: When Japanese master spy Takeo Yoshikawa arrived in Honolulu Dr. Kuhn would flash coded messages from the attic of the Kuhn household. A system that went undetected until the end.