I have trouble understanding the bold part.
Harry has a magical map by which he can see where others are. He is looking at the map and paying attention to his enemy schoolboy, Malfoy.
”Well, I’m keeping an eye on him (Malfoy) from now on,” he said firmly. “And the moment I see him lurking somewhere with Crabbe and Goyle keeping watch outside, it’ll be on with the old Invisibility Cloak and off to find out what he’s――“ He broke off as Neville entered the dormitory.
(Harry Potter and the Half-Blood Prince (6) [US Version p.388])
If I’m not much mistaken, the general meaning is “I’ll put on the old Invisibility Cloak and depart with haste to find out what he’s up to.” However, I can’t grab the make of the sentence. The problem is ‘it’ on the top of the sentence and ‘on’ and ‘off’. I thought ‘it’ meant Harry’s plan to detect Malfoy’s conspiracy at first, but in that case, I guess ‘off’ means ‘cancel’ or something. I feel it doesn’t make sense. I’m completely confused.
What do ‘it’, ‘on’ and ‘off’ mean?