Mammoth is based on a luxury apartment in New York, at the home of the young couple Ellen and Leo and their eight-year-old daughter Jackie. Leo is the creator of a successful online game and has ended up in a world of money and big decisions. Ellen is a surgeon working to save lives. Both parents are engrossed in their work, and their daughter Jackie is mostly with her Filipino nanny Gloria, which makes Ellen begin to doubt her life choices. When Leo goes on a business trip to Thailand, a process begins that has dramatic consequences for everyone involved.