Focusing on how a world of possibilities is opened up by language learners themselves, imagination has been regarded as an elusive but indispensable element in second language (L2) education. Drawing on notions of international posture (Yashima 2002), Ideal L2 Self (Dörnyei 2009), and imagined communities (Norton 2001), we theorised imagination from a psychological perspective and a sociocultural lens. Then we conducted a mixed-method investigation to examine the complex relationship between imagination and informal digital learning of English (IDLE). A total of 401 participants from a tier-one university in China answered the survey, and 15 participated in post-survey interviews. Quantitative data revealed that students' international posture could positively influence their IDLE practices. The Ideal L2 Self could both contribute to IDLE and partially mediate the relationship between international posture and IDLE. Adding nuances to quantitative results, the analysis of the interview data highlighted three major themes--imagining possible selves, negotiating access, and investing in IDLE. These themes helped to make sense of the intricate ways in which EFL learners invest in IDLE trajectories by utilising the power of imagination to negotiate their desired memberships in imagined international communities. This study concludes with pedagogical implications and suggestions for future research.