Thaddeus and Thor

The Flynn-Fletcher's neighbors have relatives visiting who bear an uncanny resemblance to Candace, Phineas, and Ferb. In fact, older sister Mandy has two younger brothers, Thaddeus and Thor, who drive her crazy with their ambitious projects. Although Candace and Mandy initially bond, they soon compete, urging their brothers to build the best backyard fort. Meanwhile, Perry finds himself in the middle of Dr. Doofenshmirtz's family reunion where Dr. Doofenshmirtz tries to win the annual kickball tournament with the Kick-inator.

Episode Summary
Linda tells Candace that she'll be going out to buy some groceries. Candace overreacts and asks her if she's in charge of Phineas and Ferb, whereas Linda replies yes. Candace walks out into their backyard and tells Phineas and Ferb not to build anything today.

Candace walks away and spots Mandy, a visiting relative of Mrs. Weaver, the Flynn-Fletchers' neighbor. Mandy tells Candace about her brothers, Thaddeus and Thor, who constantly annoy her with their inventions, while Candace talks to her about Phineas and Ferb. The two soon argue about whose brothers are even more annoying.

Thaddeus and Thor enter Phineas and Ferb's backyard and tell them that where they come from, they're known as the best. Thaddeus tells Phineas and Ferb that they'll be building a backyard fort today, and will soon outdo them.

Candace finds out about Thaddeus and Thor's competition and tries to get Phineas and Ferb to build a backyard fort that's even greater than Thaddeus and Thor's for Candace.

Meanwhile, Perry enters his lair and Major Monogram informs him that Dr. Doofenshmirtz has been sighted at Danville Park. Perry rushes over there and sees Dr. Doofenshmirtz. Dr. Doofenshmirtz tells Perry about a time in his life when his mother was inexplicably linked to kickball, and his brother Roger always outshone him.

Dr. Doofenshmirtz shows Perry his new invention, the Kick-inator 5000. He tries to get test it out with Perry but tells him it's in process when it accidentally kicks him in the face. He tries everything from adjusting the position of the ball to swinging it on a string. Finally, after many attempts, he kicks the ball.

Candace realizes that Linda has arrived in the driveway. She tells her mom that she forgot something, causing her to drive back to the grocery store while Candace is left to deal with the competition. Thaddeus and Thor reveal their fort, which turns out to have a slide and a licorice dispenser.

Candace reveals Phineas and Ferb's fort, which turns out to look just like a regular fort. Candace feels disappointed and walks out. However, Ferb presses a button and their real fort grows out of the ground, which turns out to be nearly fifty feet tall, complete with a bowling alley and a swimming pool. Thaddeus and Thor look impressed as Phineas and Ferb outshine them.

Meanwhile, Dr. Doofenshmirtz kicks the ball so far, it bounces off Phineas and Ferb's fort. As Heinz stands feet from home, gloating to his brother Roger, the ball comes back. Roger catches it and tags Dr. Doofenshmirtz out, which impresses their mother and leaves Heinz sitting in the dirt to be comforted by Perry.

Mandy leaves with Thaddeus and Thor in embarrassment. She tells Candace that Phineas and Ferb are really the best, and Candace goes outside to see their real fort. Ferb deactivates the fort (due to zoning regulations) and it sinks right into the ground just as Candace and Linda arrive outside.

Songs
Couldn't Kick My Way Into Her Heart

Perry's entrance to his lair
Through the water hose

Evil Jingle
Doofenshmirtz Family Reunion!

Background Information

 * In the Original Pitch, Linda states that most boys build forts, which was probably recycled out for this episode.
 * Similar in theme to "Cousin Bartie", a fake Season 2 episode that allegedly featured a doppelganger cousin of Phineas who would "go after Isabella."
 * Charlene Doofenshmirtz, Vanessa Doofenshmirtz, Roger Doofenshmirtz and Doofenshmirtz's Mom all appear in this episode.
 * Despite Thor bearing an uncanny resemblance to Ferb, he also bears some resemblance to Mikey from Recess and Brian from Family Guy.
 * This is the second time Doofenshmirtz builds a kicking machine.

Continuity

 * In Doofenshmirtz's backstory song, Love Händel re-appears.
 * The song, Couldn't Kick My Way Into Her Heart, is to the tune of Snuck Your Way Into My Heart.
 * In the song, one of the members of the band that sang the nemesis song reappears tackling Doofenshmirtz when he tried "kick the can".

Allusions

 * Terracotta Army: In 's fort, they have a terracotta army room.
 * The Emperor's New School:"L-O-S-E-R..." has the rhythm and same style of "K-U-Z-C-O..."

Cast

 * Vincent Martella as Phineas
 * Ashley Tisdale as Candace
 * Thomas Sangster as Ferb
 * Alyson Stoner as Isabella
 * Caroline Rhea as Mom
 * Mitchel Musso as Jeremy
 * Kelly Hu as Stacy
 * Dan Povenmire as Dr. Doofenshmirtz
 * Jeff "Swampy" Marsh as Major Monogram
 * Richard O'Brien as Dad
 * Dee Bradley Baker as Perry
 * Allison Janney as Charlene Doofenshmirtz
 * Olivia Olson as Vanessa Doofenshmirtz
 * Cloris Leachman as Heinz Doofenshmirtz's Mother
 * John O'Hurley as Roger Doofenshmirtz
 * Jaret Reddick as Danny
 * Danny Cooksey as Thaddeus
 * Aliki Theopilopoulos-Grafft as Mandy
 * (no actor is listed for Thor since he does not have any lines)