Chen Mo, a forty-five-year-old architect, is reserved and lonely. At a party, Lin Xiao, a twenty-two-year-old who is impulsive yet cheerful, unexpectedly catches his eye. Her innocent humor is like sunshine, driving away his gloom. As they meet a few more times, affection gradually grows. Chen Mo offers her small favors, unaware that beneath Lin Xiao's lively exterior lies the heavy burden of her family's financial troubles.
When Lin Xiao's family suddenly faces financial ruin, Chen Mo calmly steps in to help, while also seeing through her deepest desires. He proposes a contract: he will provide her with a comfortable life and opportunities, and in return she will accompany him as his “unacknowledged partner,” with marriage permanently off the table. After struggling with the decision, Lin Xiao accepts.
Life as a “canary” brings both sweetness and constraints. Chen Mo strictly maintains his family's boundaries, while Lin Xiao enjoys the material comforts but remains deeply uneasy. When Chen Mo falls seriously ill, Lin Xiao cares for him devotedly. He realizes how indispensable she has become, and her feelings deepen as she gives more of herself. When she cautiously asks about “the future,” Chen Mo gently but firmly restates the limits of their agreement.
When their secret is unexpectedly exposed, a storm erupts. Chen Mo faces family pressure and blame, while Lin Xiao is attacked and labeled by others. Chen Mo forcefully protects her, but also draws a clear line between her and his family. Once the upheaval subsides, their relationship is forced into even greater secrecy.
As the years pass, marriage remains out of reach. Their passion settles into a profound bond. Chen Mo becomes the pillar supporting Lin Xiao as she finds her footing, while Lin Xiao is the only place where Chen Mo can let down his guard. They transcend their contract and romantic love, becoming family to each other: impossible to define, irreplaceable, and impossible to separate. They stay by each other's side like shadows, for the rest of their lives.
