Canada's Prime Minister has lost his most important adviser. At the same time as he's drowning in scandal. But who is the key adviser walking out the door? Was Gerald Butts the puppet master behind the curtain, driven out by the SNC-Lavalin affair? Or a lifelong friend who made his exit when the right time came?

Paul Wells knows the ins and outs of Ottawa relationships better than anyone, and can take us back to Trudeau and Butts' first meeting at McGill, to a freighter in the arctic where he spent a week with the soon-to-be campaign manager, and inside the PMO where few decisions got made until the PM's best friend had made his opinion known. Is Butts' exit a sign that the SNC-Lavalin scandal is exploding? Or the first sign of a coherent response by the Liberal government? What happens now?

GUEST: Paul Wells, senior writer, Maclean's