Gu Yizhong, a scion of the Gu family in Suzhou, returns to China after completing his studies in the Architecture Department at the University of Pennsylvania. When the war breaks out, a failed rescue mission to assist his underground party girlfriend exposes all the contacts of the Suzhou underground organization. Falsely framed and accused by the Japanese collaborators, Gu Yizhong is mistaken for a traitor within the party. Unable to prove his innocence, he takes the initiative to infiltrate the Japanese collaborators and eventually becomes an undercover agent in the Suzhou special operations station of the Communist Party.