This is Sonata's curiosity and Dan Phantom's release goes in The Ultimate Enemy Returns.
[The film starts with Aryan in his human form watching Danny Phantom teaching Danielle Phantom how to use her ghost powers]
Ryan Tokisaki: Hey, Aryan.
Aryan: [looks behind him and sees Kuryan] Oh, hey.
Ryan Tokisaki: I see Danielle's getting a hand of her ghost powers.
Aryan: Yeah. Danny's been teaching her to use them for like a whole year.
Ryan Tokisaki: Yup. That's a nice necklace. How did you get this?
Aryan: My mother gave it to me. She said it allows me to turn into a human at will.
Ryan Tokisaki: Cool. And your mother is...
Aryan: Queen Athena.
Ryan Tokisaki: Wow.
[Ryan and Sonata are walking together]
Aryan: Oh look. It's Ryan and Sonata. I think Ryan did made her Key-sword.
Sonata Dusk: Ryan, have you ever wondered what Dan Phantom looks like?
Ryan F-Freeman: Sonata. I don't have to wonder. I know. He looks like Danny from the future.
Ryan Tokisaki: Only his appearance is different.
Sonata Dusk: Oh. And who is this chap with you, Kuryan?
Ryan Tokisaki: His name is Aryan. A techno-merman that Ryan met in the Little Mermaid adventure.
Sonata Dusk: Cool.
Ryan F-Freeman: Hey, Aryan. I wonder why you love the human world and your world of Atlantica?
Aryan: I saw how different they were but realized there was a way we were related.
[The three boys continue to chat, not noticing Sonata has snuck off]
Crash Bandicoot: Hey, guys.
Aryan: Oh, hey.
Ryan Tokisaki: Hey, Crash. Aryan told me about his necklace. I wonder who did Aryan rescue in the Little Mermaid adventure.
Aryan: Princess Meg.
Ryan F-Freeman: Princess Meg?
Aryan: The ruler of Equestopia.
Ryan F-Freeman: Oh. [realizes Sonata's not there] Hey, where's Sonata?
[Cut to Sonata sneaking into the Fenton lab and grabbing a Thermos (the one containing Dan Phantom) from a lab table]
Sonata Dusk: I know Danny told me not to do this. But I wanna see what Dan Phantom looks like. One little peak at his appearance can't hurt.
[She carefully opens it and gasps in awe. Just then, Danny, in his human form, arrives and sees Sonata holding the opened Thermos and gasps]
Danny Fenton: [runs up to Sonata and grabs the Thermos while she is still holding it] Sonata, what are you doing?! You were about to unleash one of my most dangerous and powerful foes!
Sonata Dusk: So what?
[They struggle as the rest of the gang arrive]
Sci-Ryan: Playing tug-of-war?
Bertram T. Monkey: With a Fenton Thermos? Not a good idea.
Danny Fenton: You're gonna break it!
[Sonata accidentally drops it]
Danny Fenton: Oh, no!
[The thermos hits the ground]
The gang: [gasp]
The gang: [sigh in relief]
[Suddenly, the word 'Released' appears on the side of the thermos and it opens]
The gang: [gasp again]
Evil Anna: Not good, this is.
[A white flame flies out of the thermos and becomes Dan Phantom himself]
Dan Phantom: [smirks] At last, after many of being stuck in that thermos, I'm free!
Ryan Tokisaki: [points his guns at Dan Phantom] Get back. I got guns and I'm not afraid to use them!
Dan Phantom: Or you'll what shoot?
Ryan Tokisaki: Mostly.
Crash Bandicoot: Do you have anything to say before he does?
Ryan Tokisaki: Yeah. Go on before I do. Spit it out.
Dan Phantom: Well, all I wanted was to help Mal gain full control of Mike. I only wanted to help! And what did he say to me?!
[A flashback to the past where Dan is with Mal]
Future Mal: Fly home, Dan. I work alone.
Dan Phantom: [voiceover] It tore me apart. But I learned an important lesson. You can't count on anyone, especially your heroes.
Mal: [playing along] Look, Dan, I was wrong to treat you that way. I'm sorry.
Ryan Tokisaki: Oh. Now Mal said sorry. I think it's because Dan is a threat. He don't know the real meaning of oppresion.
Dan Phantom: [to Mal] See? Now you respect me, because I'm a threat. That's the way it works. Turns out there's a lot of people, whole countries who want respect. And they will pay through the nose to get it. How do you think I got rich? I invented weapons. And soon, I'll have a weapon only I can defeat. And when I unleash it, I'll get--
Ryan Tokisaki: [in mind] Blah, blah, blah. Why is he toying with us. I just want to exterminate him.
[Dan is cut off when Thomas throws a piece of metal at him. Dan uses his telekensis to immoblize the gang]
Dan Phantom: [laughing, to Thomas] You sly dog! You got me monologuing. I can't believe it. It's cool, huh? Zero-point energy. I save the best inventions for myself. Also you might wanna work on your reactions. Besides, people are supposed to think before they act.
[Kuryan rolls his eyes]
Dan Phantom: Now, I'm going to change the past so I can restore my future.
Sonata Dusk: No, you can't, Danny! Ryan will stop you just like he did to Ursula!
Dan Phantom: My name is not [shouting] DANNY! It's Dan Phantom now. And oh, yes, I can change the past to restore my future.
[Ryan rolls his eyes]
Aryan: You won't, Dan! It could mess up the space-time continuum!
Dan Phantom: Oh, who cares?! [blasts a hole in the roof with his Ghost Ray] Now if you'll excuse me, [grabs Sonata] I have to go restore my future. And when I say my future, I mean my future.
Sonata Dusk: Save me, Ryanagio!
Ryan F-Freeman: I'm actually gonna give Dark Danny a five minuet head start.
Conker the Squirrel: Okay. Does that Dark Dan know about, shall we say... Kuryan?
[Dan throws them into a wall and then flies away, cackling and with Sonata still in his grasp]
Sonata Dusk: [screams in terror]