Hunsaker is a close friend of Roger Murtaugh, whom he met during the war. They fought alongside each other until the war ended, but the two men would distance themselves afterwards for unknown reasons. He served with Murtaugh in the La Drang Valley in 1965, and took a bayonet in the lungs, thus saving his life.
Many years later, in 1987, Michael Hunsaker calls the Murtaugh residence, but fails to get a hold of Roger, since they've not spoken in 12 years. Later, at the crime scene of Amanda Hunsaker's suicide, Murtaugh discovers she is Michael's daughter.
At an office building in Los Angeles, Michael is emotional in finding out his daughter was murdered according to the autopsy results which indicated she was poisoned. Murtaugh tries calming him down, but he demands that Murtaugh kill the people who murdered his daughter. As he leaves, Hunsaker tells Murtaugh he "owes" him, because he saved his life in the war at the La Drang Valley in 1965.
A couple of days later, Murtaugh meets with Hunsaker again, at Amanda's funeral. He deduces Michael knows a lot more than what he said. He reveals he had been working for a drug smuggling organization called Shadow Company. He was laundering their drug money through his bank. He was hoping to be out of it, but his daughter Amanda was killed in order to get him to cooperate. To prevent the possible murder of his other daughter Beverly and his wife Claire, he cooperates with them some more. At the same time, he reveals to Murtaugh about the next shipment and right before he could say anything more about the rest of it, a helicopter sniper shoots through the window and murders Michael. The sniper was Mr. Joshua.