Sandcastle explores the relationship between the older and younger generation of Singaporeans through the lives of one particular family. Boo focuses on the lead character, En (Joshua Tan), who plays a typical 18-year old teenager, who while waiting to enlist for National Service discovers the secret past of his late father through his grandparents – his grandfather's old videos and photo negatives kept in a box, and the remnants of past memory from his Alzheimer-ravaged grandmother.