Yet another 13th season show

Use a Grenade, What the Heck ★★★

Danny is targeting everyone in his police academy graduating class for death by grenade. Turns out it’s not really Danny doing it, but the Danno impersonator from "Welcome to our Branch Office," who went crazy while in the penitentiary, and is now out on parole. As part of the preparation for his role as the faux-Danno, he studied Danno's back story, including who was in the class. As before, both versions of Danno are played by James MacArthur. McGarrett ends up using the villains who hypnotized (the real) Danno in season eleven's "Deadly Courier," getting them temporarily out of prison to convince the Danno impersonator into thinking he’s a chicken (Little Chicken) in order to get him to give himself up and return to jail. (Thanks to Hermano of C/H/Q)