Keeping Mum

"Mummy! Mummy! Billy Martin came after me, but Grace was there and all I said was broccoli and now he's dead! Billy Martin's dead! Woohoo! Billy Martin's dead!"

—Petey

A dark 2005 British comedy starring Maggie Smith. In it the Goodfellow family is starting to fall on hard times; Walter Goodfellow, a vicar, is obsessing over writing the perfect sermon. His neglected wife Gloria is beginning an affair. Daughter Holly is bringing home increasingly unsuitable boyfriends while son Petey is bullied by his classmates.

And then, like Mary Poppins floating down from the sky, Grace Hawkins appears to set things right. Only instead of machiavellian schemes worthy of Jeeves her main strategy involves murder.

Tropes used in Keeping Mum include:

Amusing Injuries: Maggie Smith just pwned you with an iron.
Asshole Victim: Lance.
Beware the Nice Ones: Grace is very cheerful. Even when discussing multiple homicide.
Black Comedy
Catch Phrase: "It's about the flower arranging committee." Also, "Shall I put the kettle on?"
Cloudcuckoolander: Walter. "Was that a brassiere over Holly's shoulder this morning?"
Comedic Sociopathy
Dead Man's Chest: You know the one.
Death as Comedy
Died Standing Up: Mrs. Parker
Disposing of a Body: "I'll just have to use the pond again."
Dysfunctional Family: The Goodfellows to some degree.
Funny Foreigner: Lance, though YMMV as to whether he's funny or just an asshat.
Go Among Mad People: Rosie/Grace.
The Hecate Sisters: Holly the maiden (*snrk*), Gloria the mother and Grace the crone.
I Never Said It Was Poison: "How did you know about the other bodies?"
Incredibly Lame Pun: "No, not the moon. It has no atmosphere."
It's Personal: "It's the least a mother could do for her daughter!"
Karma Houdini: Grace.
Kick the Dog: Billy Martin and his cronies do some Dog-Kicking of Petey.
Luke, I Am Your Father: Grace to Gloria (which you should see coming from a mile away)
Magical Nanny: Parodied hard, especially because this film can be seen as a black comedy version of Mary Poppins.
Moment Killer: "I'll be late for work", later Gloria needing to "see to Mrs. Parker" after Walter returns from the convention. And then there's Lance's posing pouch...
Murder Is the Best Solution: Grace is introduced in this manner. As she puts it, she and her doctors often disagreed on this point.
Never Mess with Granny: Technically, "never mess with Mum" would be more appropriate, but it works, as Grace is of rather advanced age.
Oh Crap: Grace realizing that Mrs. Parker might know about Gloria and Lance; "Mum...I think you need to see this!"; the workmen announcing that the pond needs draining.
Really Gets Around: Holly. Her poor work-obsessed father can't even keep track of which boy she's on. ("This is Carl...Mark...er, David.")
Romantic False Lead: Lance.
Spoiler Title
Stealth Pun: Mrs. Parker is quite the nosy Parker, isn't she? Also, Grace, Gloria and Holly "keep mum" over something that Grace's long lost mother did...
The Vicar: Walter, of course.
Title Drop: We get close to one with Grace's "Mum's the word, dear."
X Meets Y: Mary Poppins meets Arsenic and Old Lace
Yandere: GRACE.