Milo turned a decommissioned truck into a haunted house for Halloween and invited his sister and friends to check it out. Zack states that he outgrew Halloween. The first "scare" was Diogee in the costume Brigette made for him to wear to Veggie Fest. Meanwhile, Dakota and Cavendish discuss Halloween and how it vanished out of existence sometime in the early twenty-first century. The two decide to have a good time that night and learn about the holiday. Back at the haunted house, only Sara is freaked out and it starts to move. Elliot sees the truck and rides after it on his bike. After Zack decides to see what's going on he walks out of the back, which is over a cliff, and is soon joined by Milo. The segment ends with a cliff-hanger.
Milo and Zack pull the truck to safety with a grappling hook and are met by Elliot, who says they committed almost twenty safety violations. The truck then heads downhill backwards and through eight different warehouses. Dakota realizes he lost the time grenade, then Cavendish realizes that they are the reason Halloween no longer exists in their time. So they track down the kid who now has the device - which was easier said than done.
The truck then lands in the courtyard of Jefferson County Middle School, improving the party planned by Mort and Chad. The time travelers finally find the time grenade and Dakota manages to disarm it, but not before accidentally sending him and Cavendish to Ancient Rome. Dakota succeeds in saving Halloween but inadvertently takes away February twenty-ninth so that it only occurs once every four years, creating leap years. Back in Milo's time, Zack has finally learned to enjoy Halloween again. Then his dad shows up in a fish costume. After Zack runs off, Elliot compliments Marcus' costume.
For a full transcript of "Milo Murphy's Halloween Scream-a-Torium!", click here.
In the initial storyboards, Milo was going to be a zombie.  In the episode Milo makes a comment about deciding not to go with the zombie costume. This may be referencing that he originally was going to dress up as a zombie.
On the telephone poles that take off the truck's mirrors there's a poster of a missing bike. It may be a running joke from the missing bike posters in the Phineas and Ferb episode Summer Belongs to You!.
Many of the extras seen trick-or-treating also appeared in various Phineas and Ferb episodes. Some notable examples are:
Young Frankenstein: The way Vinnie Dakota pronounces "Frankenstein" in Tonight's The Last Halloween is similar to how Dr. Frederick Frankenstein wants his name to be pronounced in the film. Instead of "Frank-en-stine", he pronounces it "Frank-en-steen".
IT - Brick resembles Pennywise the Clown after emerging from the snow.
The Rings - Savannah resembles the main antagonist of the movie after walking out of the snow.
Little Red Riding Hood - During Tonight's The Last Halloween, Dakota is dressed up as Little Red Riding Hood and Cavendish dressed up as the wolf.
Groucho glasses - Cavendish and Dakota used Groucho glasses as their first costumes at an elderly woman's house.
Paul Bunyan - Dakota dressed up as a lumberjack and Cavendish dressed up as an ox while as one of their various costumes to trick or treat at the same house.
Little Bo Peep - During Tonight's The Last Halloween, Cavendish is dressed up as Little Bo Peep and Dakota dressed up as a sheep.
When Vinnie is singing on the porch of the first house, as he turns his head after saying, "What a racket?" the iris of his left eye remains stationary for a moment before snapping back into his eye.
When Melissa says "Feels like we're really moving" while the truck is teetering on the cliff, she says it in Sara's voice.