McGarrett and Danny pose as drug pilots in a plan to stop a meth epidemic on the island; Danny is forced to break cover and land the plane when McGarrett is seriously wounded.
When a college professor is killed, Five-0 goes back to school with Danny, who is posing as the professor's replacement.
A scuba diver is killed with a gun that was used in a recent murder; McGarrett gets ready to propose to Catherine.
The team discovers a Russian spy ring hiding on Oahu when a Russian submarine appears off the coast.
When a deliberate chemical spill enables six dangerous inmates to escape, the team goes on an intensive manhunt to find them.