This is the second episode in a special two-parter. Debby has just been murdered and the police are hot on any lead they can find. Buck, believing that his wife Ms. Liz has committed the crime, starts planting evidence to make it seem as if the one true Hank committed the heinous crime. Hank does not have an alibi as he was with Debby's roommate Gale and had accidentally smoked marijuana in attempting to light a cigarette with a joint. Hank believes he may have blacked out and may have committed the crime, as he is unaware of the effects of marijuana. Hank tries furiously through the episode to find out what exactly happened to him and hopefully he can clear his name. He eventually become aware that he accidentally smoked the marijuana after Debby was already dead so he couldn't be responsible for the crime. In an attempt to wrap this part up, Hank let's Gale be arrested for the murder so he doesn't have to admit that he and Gale were smoking weed and couldn't be the murderers. While the town jokes about putting a lethal injection needle in Gale's arm, Hank finally admits, through great shame to the whole community, that he and Gale were innocent because they were 'smoking pot' and though he didn't enjoy it, the wrong man was in jail.
This whole episode only works because of how the drug laws were setup. The demonization of marijuana that initially lead to it's criminalization programmed generation of people to believe marijuana has devilish effects on people (you can see examples of that on this very site). Hank holds these beliefs and it makes him first believe he was capable of murdering someone while stoned. Stoned people don't murder people, murderers sometimes gets stoned. Had Hank lived in a society where marijuana wasn't a schedule 1 drug, he might have known that he's more likely to murder someone after taking a sip of beer than a sweet cherry toke. Moving past this, Hank's shame of having smoked let him allow an innocent man (Gale is a man) sit in jail for at least a night. An innocent man being deprived of his rights because Hank is worried about the social perception of him smoking marijuana. That is a great injustice. Luckily, he overcomes his fears and is open with the justice system.