The Laws of Motion is the tenth and final episode of the second season of Father Brown.
A day out at a local hill climb track ends with one of the drivers killed in a car crash. When Father Brown won't stop investigating, an infuriated Sullivan resolves to put an end to his meddling for good.
Tropes:
- Asshole Victim: Audrey MacMurray, who antagonises enough people in her final few hours that no less than three separate people make an attempt on her life — one planned, two completely spur of the moment.
- Beautiful All Along: Phyllis's bookish look transforms when she does the classic The Glasses Gotta Go and Letting Her Hair Down to become a sultry sex kitten for Walter.
- Career-Ending Injury: Walter MacMurray's football career ended prematurely after he suffered from a knee injury, making him very amenable to marrying the very wealthy Audrey.
- Make It Look Like an Accident: The plot of the killers involved staging a car accident to mask the fact that Audrey was killed before the race began. What they didn't count on was someone else tampering with her car in their own attempt to cause the her to get into a fatal accident.
- May–December Romance: Audrey (December) and Walter (May) MacMurray. Even Lady Felicia congratulates Audrey on being able to snag such a looker. Meanwhile, Mrs McCarthy remarks that she's old enough to be his mother.
- Mrs. Robinson: Her love of young men doesn't stop there — she's also having an affair with Gary, and even seduces Sid while he's in the men's room.
- Shout-Out: To Double Indemnity, in that the killers successfully conspire to murder one conspirator's spouse.
- Trophy Husband: It's clear to everyone but Walter MacMurray that his wife Audrey only married him to have a pretty young thing to show off to her peers.
- Vehicular Sabotage: Audrey has the brake lines of her car cut while she is participating in a hill climb.
- Wrench Whack: Audrey is done in with a blow to the head from a lug wrench. The killers then plan to make it look like she died in a car accident.
