Mau lives with Sue in the shed at Tokyo Station alongside the old steam locomotive Yaemon. Their combined names form a Japanese approximation of the English word "mouse". Mau and Sue initially evade Summer, a cat attempting to prey on them. When Yaemon protects the mice, Mau and Sue seek to repay him. Learning Yaemon faces scrapping due to shed demolition plans, Mau joins the effort to restore the locomotive.

Mau assists in repairing Yaemon, helping remove rust and procure coal by snatching it from a train bound for a power plant. Mau is present when the mice deliver a speech to Yaemon, reigniting his determination and sparking a literal "fire in his heart". During the subsequent escape, Mau helps unhook Yaemon from Fukku's crane, triggering a muted-color flashback sequence for Yaemon recalling the repairs.

Mau operates primarily as part of a duo with Sue throughout these events. The character appears exclusively in the 2009 animated adaptation.