Flash is a Siby Malayil family thriller. It a story about a girl who is born and brought up in Chennai. There is little that she knows about other relatives in Kerala or about the life, culture and traditions of the State. When her mother dies of cancer, she is brought to the ancestral home. Her grandfather dotes on her to make up for the years of separation and lavishes immense love and affection on her. It is at this juncture that Mithun Madhavan (Mohanlal) comes into her life, following which everything changes.