Mum and Dad are not related by blood to Birdie and Elbie but have taken them under their wing and consider them part of their family. The four live in a cramped and dingy apartment in a seedy corner of the airport, where they scavenge for food and other essentials.
The movie follows the dysfunctional family as they go about their mundane lives, punctuated by occasional bursts of violence and depravity. Mum and Dad are sadistic and sexually deviant, using their authority at the airport to indulge their darkest desires.
When Lena, a meek and lonely Polish woman, starts working as a cleaner at the airport, she strikes up an unlikely friendship with Birdie. As Lena becomes more involved with the family, she learns the true nature of their twisted relationship and the horrors they are capable of.
As the movie progresses, the violence and gore escalate, culminating in a shocking and brutal finale. Mum & Dad is a bleak and unflinching portrayal of depravity and dysfunction, with no heroes or happy endings.