Tony's plan was this: After Corrigan should be asleep he (Tony) and Burney would cut the mooring ropes that held the boat to the shore.
(Tony's plan; was; this: After Corrigan should be asleep) (he (Tony) and Burney; would cut; the mooring ropes that held the boat to the shore) (the mooring ropes; held; the boat; to the shore)
Tony's plan he (Tony) and Burney the mooring ropes
was would cut held
this: After Corrigan should be asleep
the mooring ropes that held the boat to the shore
the boat to the shore