Ethan is a bright young boy obsessed with physics and astronomy who lives with his mother, Claire. Claire has been a wonderful parent to Ethan, but he's becoming increasingly curious about his father's absence in his life, wanting to fill in the gaps.
Claire's life is centred on Ethan; she is fiercely protective of her talented, vulnerable son, and of her own feelings. When Ethan falls ill, tied to a tragic event from when he was a baby, Claire's tightly held world is split open.
On the other side of the country, Mark is trying to forget about the events that tore his family apart. Then a sudden and unexpected call home forces him to confront his past, and the hole in his life that was once filled with his wife Claire and his son Ethan.
When Ethan secretly intercepts a letter from Mark to Claire, he unleashes long-suppressed forces that - like gravity - pull the three together again, testing the limits of love and forgiveness.