One key event is the flashback to the Jallianwala Bagh massacre. It sets the tone for Sardar Udham's quest for revenge. Another is his time in London, where he carefully plans his move against the man responsible for the massacre. And finally, his act of shooting the target in London is a significant event that is the culmination of his years of planning.
If 'Sardar based on true story' is a movie, perhaps it tells the story of Sardar's life journey. It could involve his achievements, struggles, and how he overcame various difficulties. Maybe it shows his contributions to a certain community or cause. However, without seeing it or having more details, it's all speculation.