While on the subway on a school field trip, Milo’s car gets thrown off course and he, Zack and Melissa discover an underground city of subterranean creatures.

When Milo accidentally joins another family on their vacation, the new family must learn to cope with Murphy’s Law.

At a donation drive, Milo accidentally gives away his mother's vintage T-shirts. Meanwhile, Cavendish and Dakota are assigned to protect the pistachios at the zoo.

When Cavendish and Dakota stop Milo from destroying a pistachio plant, they change the future. Pistachio plants become sentient and take over the planet, so Cavendish, Dakota and Milo must race through time to save the future.

Elliot Decker is hired as the school’s head safety monitor.

Parents become children at Back to School Night.

Dakota travels back in time to save Cavendish.

Milo, Zack, and Melissa—with help from Phineas, Ferb, Candace, Buford, and Baljeet—help conquer Murphy's law to stop a swarm of Pistachions.

Doofenshmirtz reminisces on an adventure he had with Diogee the previous summer after the canine got caught up in an Agent P style mission.

Learning that the local skating rink is closing down, Milo convinces his parents to pursue their dream of winning a disco skating competition they were unable to finish years ago before that happens. Elsewhere, Cavendish and Dakota are assigned to pick up seemingly ordinary lawn gnomes from a little old lady.

While Milo is sick, two shapeshifting aliens attempt to get a tissue sample. Meanwhile, under Monogram's orders, Agent P continues to keep Doof from doing harm while attempting to do good.

Milo gets abducted by the recurring alien characters introduced this season, while Melissa, Zack, and Doof attempt to look for him. While on trash duty, Cavendish sees the abduction from a distance, but Dakota does not. This causes a large argument between them, which ultimately ends in the dissolution of their partnership and the former going rogue.

While attending a school award ceremony, Milo attempts to get to the stage in time to collect an award he was nominated for. Meanwhile, Diogee ends up riding along with two of Danville's police officers while they attempt to catch Zippy, the world's fastest koala, and rescue one of their K-9 officers.

After the school football field is ruined, an event that was meant to be there gets transferred to an old dilapidated park, where Melissa and Amanda compete over who can fix it up faster. Meanwhile, Dakota and Doof deal with an alien device that turns bones into a noodle-like substance.