The recent dynamic turn in second language acquisition research has called for an investigation in learner agency by taking its complex dynamic nature into account. Informed by complex dynamic systems theory (CDST), this study investigated the agency of learners in a complex educational context where mainstream schooling and private tutoring (shadow education) coexist, and when teaching and learning was switched online. Through longitudinal narrative inquiry, this study analyzed the experiences of three senior secondary students enrolled in online English private tutoring. Data were collected through three rounds of individual interview and two pieces of learner reflective writing, supplemented by artifacts such as participants' language-learning materials and interviews with their tutors, schoolteachers, and parents. The multiple sources of data collected for 1 year were compiled as narratives for analysis. The findings highlight the importance of considering learner agency from the perspective of CDST, acknowledging characteristics such as its relational, ecological, emergent nature, and its spatiality, multidimensionality, and sustainability. This study sheds light on the complex agency-structure interplay in shadow education situated in the wider education context, and offers implications for educators to support language learners to be agentic in regulating their learning.