A thousand-year-old snake demon sought out a human CEO because of a snake immortal mark, and her identity was exposed after one night. Instead, he took her in, living together sweetly, until a demon realm field agent appeared, forcing her back for violating human-demon love laws. Her inner elixir was unstable, and staying in the human realm would cause her to lose her memory, so she had no choice but to leave. He chased into the demon realm and discovered that the two had already signed a symbiosis pact letter of intent on the night they were drunk. Her memories blurred day by day, and he chased time and time again, arriving a step late each time. She was taken away for memory erasure, and he went to stop it. Her inner elixir backfired and she fainted, so he signed a symbiosis pact trading his lifespan in exchange for her life, and white hair grew at his temples. After waking up, she split half of her natal demon power, turning the contract into a two-way shared longevity. In the end, the field agent let go, and the two held a wedding.