I believe this is the last episode of the Inspector Lewis series as the characters' lives seem to be set in various directions.
I'll go on record now and state that this two-part episode (which I saw as one part) features one of my favorite "MI-5" actresses, Miranda Raison, and she gives a good performance here as a student/colleague of the murdered man.
The story actually goes back some years, when a young woman was struck by a drunk driver and killed. The driver, a university scientist, goes to prison. When he's released, he's found dead, having been hit by a car, but he's managed to scratch the number 500 on the car before he expires.
Lewis and Hathaway speak with the sister and mother of the dead woman, and also the wife of the dead man who, strangely, did not hear this awful accident that took place right outside her window.
Lewis & Hathaway find themselves involved in Oxford University politics, find a body long dead, discover the secret of "300," and deal with a suicide and another murder before realizing the identity of the culprit.
I of course knew who the culprit was because I had this on DVD, and when I entered the room, my sisters were watching the very end and discussing the murderer. So I can't say whether or not the murderer was obvious. I'm guessing not. There were suspects aplenty.
Edward Fox, Laurence Fox's (Hathaway's) uncle, makes an appearance.
I thought this was a good episode and a fitting ending to the series. It's an absorbing mystery.
Like many, I feel a connection to Lewis due to the Inspector Morse series. We are being regaled next week with another "Endeavor" episode featuring the young Inspector Morse. So efforts to keep the Morse character going continue. Unfortunately I can't connect the Endeavor series with John Thaw or the older Morse. At all. But I'll probably watch it.