Gu Chang'an inherits the Chang'an Lane old building from his elder brother Gu Changping, but receives a notice that the building will be auctioned off in ninety days. He teams up with tenants Ye Tang, Su Rui, Lin Zhi, and lawyer's daughter Gu Nian to launch an investigation, opposing the entrusted legal consultant Pei Jin. Relying on physical evidence such as the B17 key, everyone digs deep into the truth of the old warehouse fire five years ago, cracks the traps of forged documents and identity theft, and clarifies the origins of the confidentiality agreement and relief funds. After various twists and turns, Pei Jin's conspiracy is exposed, and the case is transferred to criminal investigation. Chang'an Lane is listed as a protective building, and Gu Chang'an steps down from his identity as a landlord, taking the lead as a resident representative to establish a residents' cooperative, guarding the old building together with everyone.