D irected by Patrick Leung, this affecting saga from Hong Kong is a bit tricksy to get to grips with because it keeps hopping back and forth between an assortment of time frames. It tracks a set of characters as children in the mid-1980s, played by one group of young actors, and then later in the 1990s and early 00s when an adult cast takes over. But as it spirals in towards its surprising and dramatic conclusion, everything falls into place and the last 10 minutes is properly tear-jerking – even if it’s unabashedly sentimental, like a classic melodrama. The key incident foretold in the title happens around halfway through, although it’s no spoiler to know it’s coming. In 1985, a quartet of kids growing up on a working-class Hong Kong housing estate – boys Kin (Matthew Wong Cheuk-yin) and Heem (Chui Ka-him), and girl Hoyi (Lam Seung-yu) and her kid brother (Shawn Heung Sung-yu), for ever called Little Brother – see a UFO in the sky one night.…