Phineas and Ferb throw Isabella a suprise party, but all Isabella wants is some quiet time with Phineas. Meanwhile, Stacy witnesses a battle between Doofenshmirtz and Perry, and discovers his identity.
This episode contains the following tropes:
- Ageless Birthday Episode: This time for Isabella, as the title says.
- Bad "Bad Acting": Stacy when speaking with Monogram speaks very stiltedly when claiming to have come home from the mall.
- Birthday Episode: For Isabella, obviously.
- Buffy Speak: Monogram, disguised as a member of the Danville Department of Water and Power, says he and the others are at Stacy's to "read your gas... thingy."
- The Bus Came Back: One of the attendees for Isabella's birthday is Melissa from the Li'l Sparks from "We Call It Maze".
- Call-Back: Perry gives Stacy OWCA's "So You Discovered Your Pet is a Secret Agent" pamphlet, only this time, Stacy actually reads it, and this is how she discovers the Loophole Abuse and becomes his Secret Secret-Keeper.
- A Day in the Limelight: The title of the episode may be titled "Happy Birthday, Isabella!", but the B-plot is pretty much focused on Stacy.
- Dumbass Has a Point: When Perry is fighting Doofenshmirtz on his bus, Doofenshmirtz points out how there is nobody driving the vehicle. On cue, it goes out of control.
- Earn Your Happy Ending: After spending a birthday party with everyone else, Isabella finally gets what she deserved and has an ice cream meal with Phineas at the Soda Fountain.
- Everyone Has Standards: Even if Candace is way too obsessed about busting her brothers, she’s not going to bust them for sprinkling confetti and multiple piñatas. She only tries to bust them when it gets to the grand finale.
- Evil Is Petty: Similar to his scheme from "Mom's Birthday", Doof creates an inator to get rid of all of the little things that annoy him by turning them into bugs.
- The Friends Who Never Hang: Candace and Holly interact with each other for the entirety of Isabella's party.
- Funny Background Event: During Isabella's song, the rest of the gang get some Noodle Implements for her surprise, including a giant shoelace and the inexplicable Giant Floating Baby Head.
- Green Aesop: OWCA has installed motion-sensor lights for Perry's lair and Monogram's office. Unfortunately, they have to be in constant motion for the lights to stay on.
- Halfway Plot Switch: The Doof & Perry plot starts out normally, but then Stacy finds out Perry's secret and whatever the scheme was supposed to be is all but forgotten.
- Hidden Depths: Turns out that Stacy is a huge fan of horror films. This interest is very clearly setting up for her role as Captain Oblivious in "Night of the Living Pharmacists".
- I Am Not Weasel: Once again, Doof gets mistaken (by Stacy) for a pharmacist, but this time, Doof gets annoyed.Stacy: And who is this pharmacist?Doofenshmirtz: Really? Really? W-Why does everyone just stampede to that conclusion? Other people wear lab coats, you know. Why don't you say, "Oh, look! A rocket scientist!" Or "a veterinarian"?Stacy: Are you a veterinarian?Doofenshmirtz: No! But that's not the point!
- I Need to Go Iron My Dog:Phineas: Well to make it up to you, it's kinda lame but... maybe Ferb and I could, I don't know, take you out for some ice cream or something?
Isabella: Uh-
Ferb: I... just remembered there's... something I need to do. You two go. - "I Want" Song: Isabella expresses her desire to spend time alone with Phineas as her present.
- Jumping Out of a Cake: Baljeet expresses hope that a... "male dancer" does not pop out of the giant red velvet cake.Buford: They asked, but they couldn't afford me!
- Loophole Abuse: Stacy gets to keep the knowledge of Perry's secret identity and Perry doesn't have to leave the Flynn-Fletchers because OWCA's rules only apply to the agents' host family, which Stacy isn't a part of.
- Losing a Shoe in the Struggle: Doof's inator accidentally turns his left shoe and sock into bugs and he spends the rest of the episode with one bare foot.
- Running Gag: Buford stuffing Isabella into a sack.
- Secret-Keeper: Stacy learns of Perry's secret identity and doesn't get her mind erased.
- Shout-Out: The horror movie Stacy watches, The Grievance, is a parody of The Grudge. Complete with a greasy Stringy-Haired Ghost Girl that Stacy inadvertedly takes the appearance of when she slips on an oil slick from Doof's -inator.
- Spoonerism: Doof tells Perry not to attempt to go through his protective bubble because it's "Platyproof-poof".
- Suspiciously Specific Denial: During the credits, Candace is spying on her brothers and noting Phineas wondering where Perry is Once an Episode, she starts to ponder where Perry is herself. Stacy denies any knowledge of what Perry is doing and makes it clear she knows exactly what Perry is doing, but Candace is too focused on her brothers to notice.
- Take Our Word for It: When the grand finale is revealed, we only see it as a shadow and only the guests’ reactions are shown before it gets turned into butterflies by Doof’s -inator.
- This Is the Part Where...: When Phineas is unveiling the grand finale:Holly: Um, this might be the time to call.
Candace: Yup! - The Unreveal: We never see the grand finale for Isabella's birthday, as Doof's -inator turns it into a kaleidoscope of butterflies.
- Wham Episode: Stacy learns of Perry's secret double life and promises to keep it to herself. (Although, this is not brought up in future episodes, as the series ends before it can.)
