The episode was about a man who had been falsely accused (and convicted) of murdering his wife. Another murder occurred in his residence and he was blamed. Luther who was responsible for his conviction now defends him. They did a great job setting it up and displaying Luther's involvement.
Luther began believing he was write getting him convicted of the murder 15 years back and doesn't want any problems. The man comes to his office to ask for his help with this new problem, his landlord's death. He says he has gotten over what happened in the past, he just wants his help.
Luther wants no part in it, however the rest of the team wants to take on the case, especially Ron. When Ron is asked later by Tom why he is doing it for free Ron tells him it is an investment... an investment in Luther. He sees it as an opportunity for growth for Luther which will keep him on the side of the defense and make it unable for Luther to go back to prosecution.
As Luther looks into the case forced he begins to question if the man was guilty for the murder of his wife and later finds out he was innocent. This begins to tear him apart. He fights harder to keep this from happening for a second time and wins.
This episode is turning point for Luther. I look forward to seeing what he does in future episodes.