"A Christmas Peril" is the twenty-first episode of Milo Murphy's Law, and also the series' Christmas episode. It premiered on December 2, 2017 and is the last episode of the first season in airing order.
The episode opens on Christmas Eve in present-era Danville, where Milo and Sara are driving through town in their parents' car, accompanied by Melissa and Zack. Milo explains to his friends that, as his Christmas gift for his parents, he has invited the extended Murphy family over to spend Christmas together, which they haven't done in years due to how catastrophic their mishaps can get. Just then, they hear a weather report saying that a snowstorm is expected to hit later tonight, and Sara remarks that it's a shame their car cannot simply fly over the snow.
Meanwhile, in the far-off year of 2195, Cavendish is at the mall, seeing that Dakota has now become a famous pop singer and is signing copies of his autobiography nearby. Cavendish reflects on how, 20 years earlier, at the Bureau of Time Travel's Christmas party, he and Dakota got into a very bitter argument because Dakota ate the last egg roll at the buffet table and ended their partnership as a result. Cavendish deeply regrets having driven away his best friend and, upon remembering he's a time traveler, decides to go back to the 2175 to prevent the argument from ever happening (and he ends up bringing along several other elderly gentlemen who wanted to come with him), even though he knows he'd be breaking a lot of rules in doing so. Unbeknownst to Cavendish, though, Dakota also regrets the argument that ended their partnership and also decides to travel back to 2175.
Upon arriving in 2175, the older Cavendish sneaks in to the Time Bureau's Christmas party and tries to prevent his and Dakota's younger selves from reaching the buffet table by changing the sign pointing to it. Unfortunately, after he has done this, the older Dakota, disguised in a Santa Claus suit, does the same thing, thus accidentally undoing Cavendish's switch and causing the argument to happen anyway. Older Cavendish spots Older Dakota and the two argue over who should stay out of the other's way as their younger selves break up.
Back in the present-day, the group's first stop is the airport to pick up Milo and Sara's older cousin, Nate, who insists that he is not affected by Murphy's Law, claiming it skipped a generation for him. Despite his claims, however, he is shown to have lost his luggage, which is mistakenly being loaded onto another plane. Milo and Nate chase after the suitcase, but end up accidentally boarding the plane in question; and to add insult to injury, Nate's suitcase ends up on the plane's wing, only to fall into the ocean and get eaten by a shark. Eventually, Milo and Nate catch back up with Sara and the others by parachute. Next, the group goes to the train station to pick up Grandma and Grandpa Murphy, who have a habit of talking over each other. When Milo and Nate step into the train to help their grandparents carry their luggage, the doors shut behind them, and the train takes off for Canada. Luckily for the Murphys, the train suddenly breaks off the tracks near the harbor, where Nate's parents, Joey and Laura are arriving by boat. Before Joey and Laura can get off the boat, though, it is hit by an iceberg and starts sinking, so Milo takes control of a nearby rowboat to go save his uncle and aunt.
Older Cavendish makes another attempt to fix his mistake, this time deciding to snatch the last egg roll before his and Dakota's younger selves reach the buffet table. Older Dakota has the same idea, however, but in his case, he brings in a Velociraptor to eat the egg roll. This unfortunately fails, and again, Younger Dakota eats the egg roll and breaks up with his partner. Older Cavendish travels back once again, this time dressing as a giant egg roll and claiming he was turned into one because the egg rolls are tainted. But in doing so, he is attacked by the Velociraptor that Older Dakota brought along in his previous attempt. Meanwhile, Older Dakota tries destroying the roof with Hamosaur, but this somehow fails to distract the younger Cavendish and Dakota from their partnership-ending argument.
In the present, the Murphys all gather into Martin and Brigette's car, but they soon see that the blizzard mentioned on the radio is starting to come down hard, and it quickly overtakes the whole car. Milo starts to think he may have had the wrong idea in getting his entire jinxed family together, but after hearing his relatives reminisce about their past Christmas gatherings, and Sara saying they can surely find a way out, he gets back in the spirit. The car's engine is frozen, though, but luckily, Diogee points out a nearby horse-drawn carriage. The man operating the carriage is happy to give them a lift, but then the carriage slips onto a frozen lake, gets detached from the horse, and flies into a tree with everyone still in it. Melissa points out a building nearby, so everyone climbs down from the tree and make their way towards it.
Back in 2175, Older Cavendish and Older Dakota ask each other what they're doing here and then realize that they're both trying to save their friendship. They decide that they might have a better chance of stopping the original argument from happening if they work together. However, they try to do this by wrecking the party, and their multiple attempts to do so (using battle suits, a pack of Velociraptors, plant monsters in Santa suits, and driving a pirate ship through the wall) all fail to prevent the argument from happening. After many of these failed attempts, Older Cavendish and Older Dakota simply approach their younger selves, telling them to stop their arguing. (Mr. Block briefly interjects to point out how many rules their older selves have just violated, but he is stopped by his older self.) Upon learning what their older selves went through trying to save their friendship, the younger Cavendish and Dakota reconcile, and as a result, all of their older selves vanish due to the timeline having changed.
Back in Milo's time, Martin and Brigette are at the mall and are informed that the shoppers will have to stay inside the mall due to the blizzard outside. Just then, Milo and the others show up, as the mall happens to be the building that Melissa pointed them to. Martin and Brigette are pleasantly surprised that Milo brought the whole family together, but since they cannot leave the mall due to the snowstorm, they instead decide to have their Christmas Eve dinner at the mall's Chinese restaurant (during which Melissa's dad, Zack's parents, and Amanda and Mort join in). At a separate table nearby, Cavendish and Dakota insist on giving each other the last egg roll. The episode then ends by revealing that, in the newly-changed timeline, Cavendish and Dakota remain together and become a pop-singing duo.
- "S'Winter" (instrumental version)
- "We're Going to the Zoo"
- "Candace Party" (instrumental version)
- "This is Happening Right Now"
- "Everybody is Here"
- This episode first aired in France on November 5, 2017.
- An instrumental version of the song "S'Winter" from the Phineas and Ferb episode of the same name can be heard at the beginning of this episode. The instrumental version of "Candace Party" from "Candace Gets Busted" is also used when Milo and Nate are chasing Nate's luggage at the airport.