S01E55: Moominpappa's Second Youth

One day something remarkable happens in Moominvalley, Moominpappa goes for a walk without his hat. When made aware of this he is greatly distressed, fearing that his forgetfulness is a sign of approaching senility. he takes up the most strenuous physical exercises to regain his lost faculties, and makes day into night in an attempt to get out of the ruts of his life. To get him back to normal takes quite a jolt; a very gently one which is administered by Moominmamma - who else ?