Margaret abandoned her family soon after her son was born, and she now has been out of their lives for 18 years. When news about her son's disappearance reaches her, not much sorrow is demonstrated. Nevertheless, around the same time, she accepts a wild kid from the street, roughly the same age as her own child, to live with her. The big question obviously being why. A simple act of generosity? Or perhaps another shot at maternity, after foregoing that responsibility all those years ago? For a while, the latter option seems to be the case, but when things get overly physical between her and the boy, Joe, that theory doesn't hold up any more. If motherhood is indeed Margaret's objective, she has some odd notions of the concept at least.
Unfortunately, Mammal - the title suggests a nurturing nature to their relationship based on maternal instincts, though there's also an undeniable social aspect to it as well, so one can look at it from both angles - is short on motivations. It's not Daly's intention to spoon feed us all the answers, which is fine, but there's simply too few of those concerning the various characters' actions to go around. Things happen as they do, while particular reasons are entirely up to the viewer to come up with. It makes Mammal a rather hollow film. Thankfully, there's strong performances throughout, which do make us care enough to stick with the protagonists rather than lose all interest entirely. We hardly get to know these people to the extent that we should for Mammal to deliver the gripping drama it feels like it wants to, but as fellow mammals we sympathize enough to feel some emotional connection to stick with them for a good hour and a half.