Drake, having lost a coin toss to Herb Muddlefoot, is returning from having taken Gosalyn, Tank, and Honker to a rock concert, the Hamburger Hippo, and the video arcade, at the last of which Gosalyn won a blueberry jawbreaker (which Tank unsuccessfully tries to steal for his collection). Gosalyn's attempts to persuade Drake that he needs to trust her to make her own decisions about how to spend her leisure time are interrupted by the crash of a cucumber-shaped spacecraft. The door of the craft opens to reveal a small, purple, seemingly benign alien wearing a collar that he seems keen to have removed. Tank, meanwhile, finds a cucumber-shaped pouch containing what look like blueberry jawbreakers, which he empties into Honker's backpack and replaces with gravel when the alien tries to stare him down into returning the pouch. The alien's doglike behaviour convinces Gosalyn that he is some sort of pet, and she insists that she and a sceptical Drake look after him until his owner arrives.
Back at the Mallard residence, while Launchpad regales an uninterested Drake with the story of the strange adventure he has just had, the alien is horrified to discover the evidence of Tank's theft, and even more horrified when Gosalyn fires up a blowtorch to remove his collar. Her second choice of implement, a chainsaw, causes the alien to inflate like a balloon and crash through the roof; when Drake pokes him with a pin, he climbs through the hole and uses his extendable eye stalks to spy on the Muddlefoots, where he sees Tank with the backpack of "jawbreakers". He launches himself over to the Muddlefoot house using the Mallards' TV antenna, and tries to inflate himself to giant size to attack Tank for the theft, but the collar causes him to shrink back again. Gosalyn arrives and kicks Tank into a wall, allowing the alien to recover Honker's backpack, unaware that Tank has emptied the "jawbreakers" into a glass jar to add to his collection. When Honker attempts to ask for the return of the backpack, as it contains his homework, the alien inflates and throws him across the street. As Gosalyn berates Drake for not trusting her, a cabbage-shaped spaceship lands on their front lawn; the terrified alien flees into the next room, while Drake is knocked flying as the front door is broken in by a large robot who tells Gosalyn he has come for Wacko. Gosalyn asks who or what a "Wacko" is, and the robot grabs the alien and identifies him as Wacko, then leaves in search of "seismospheres".
Drake dons his Darkwing costume and tries to intervene; Gosalyn is more successful, pulling the robot's visor over his eyes and hurling Wacko at the spaceship. An annoyed Darkwing sends Gosalyn to her room, and he and Launchpad fight the robot as Gosalyn ties her sheets together and climbs out of her window, then grabs Honker and boards the spaceship. Darkwing and Launchpad are outmatched by the robot, who identifies himself as Big Nasty, captain of the Outer Space Patrol. Darkwing is alarmed to learn that Wacko is not a pet, but a dangerous criminal in possession of the most deadly explosives in the universe, and he is even more alarmed to see Gosalyn waving from the window of Big Nasty's ship as it lifts off. He, Launchpad, and Big Nasty give chase in the Thunderquack. Meanwhile, Gosalyn finds a machine aboard the spaceship that removes Wacko's collar, restoring him to his usual muscular physique and allowing him to speak again. However, he finally discovers that Tank mistook his seismospheres for jawbreakers and that they are no longer in Honker's backpack, so they turn the ship around to collect them. Tank, meanwhile, discovers the difficult way that the seismospheres are powerful explosives when he throws a tiny one aside and literally blows the roof off his family's house.
Big Nasty's ship sputters to a halt, allowing the Thunderquack to catch up; Wacko disguises himself as a pillar as Darkwing, Big Nasty, and Launchpad enter and demand Wacko's whereabouts. Gosalyn, who still resents Darkwing for not trusting her, refuses to tell him, then encourages Wacko to make a break for it in the Thunderquack. Darkwing finally explains that Wacko is a dangerous criminal, which Gosalyn doesn't believe, even as she hears his maniacal laughter coming from the Thunderquack before it speeds off, causing an explosive decompression of Big Nasty's ship that nearly sucks Gosalyn into space before Big Nasty plugs the hole with a giant suction cup. Gosalyn continues to insist that Wacko is harmless until a seismosphere falls out of Honker's pocket, having been mistaken for one of Tank's jawbreakers by the younger Muddlefoot. Big Nasty says they can use the seismosphere to jump start his ship, and Darkwing dons a spacesuit to fire it into the thrusters, although he struggles to actually hit the target until the ship enters the atmosphere and he can remove the suit and fire the sphere from a peashooter. Meanwhile, Wacko crashes the Thunderquack in the Muddlefoots' garden and gets a heavily bandaged Tank to tell him that the seismospheres are in his closet.
Darkwing and Big Nasty show up before Wacko can make his getaway, but he threatens to detonate the seismospheres unless they let him leave. Gosalyn is finally convinced that Wacko is evil when she overhears him announce his plans to lay waste the universe, and she pokes him in the eyes and allows Darkwing to subdue him and replace his collar. With Wacko re-captured, Big Nasty takes off in his ship. As Honker returns home, Herb Muddlefoot thanks Drake for looking after his kids; as he slams his front door, his house, structurally weakened by the seismosphere explosion, promptly collapses.
This episode contains examples of the following tropes:
- Ax-Crazy: Given a chance, Wacko will destroy the universe using his explosive seismospheres, and he will smile ear to ear as he does it.
- Bandage Mummy: After he is blown up by the explosion of the tiniest of Wacko's seismospheres, Tank ends up in a full body cast; even his mouth is covered, muffling his revelation that the rest of the spheres are in his closet.
- Bedsheet Ladder: When Darkwing sends Gosalyn to her room during the initial confrontation with Big Nasty, she simply ties her sheets together and climbs out of her window to help Wacko escape in Big Nasty's ship.
- Continuous Decompression: When Wacko speeds off in the Thunderquack, leaving a giant hole in the side of Big Nasty's ship, the contents are sucked out in a powerful yet steady stream until Big Nasty is able to cover the hole with a suction cup.
- Evil Laugh: Wacko lets out a particularly maniacal laugh as he takes off in the Thunderquack after persuading Gosalyn to help him remove his prisoner's collar.
- Eye Poke: When Gosalyn finally gets incontrovertible evidence that Wacko is evil and he sarcastically thanks her for helping him with his destructive plans, she gives him a poke in both eyes to distract him for long enough for Darkwing to get the upper hand in a one-on-one fight.
- Furry Confusion: Upon returning from a bizarre adventure including the pet shop, the bowling alley, and the St. Canard ballet school, Launchpad has a bird cage on his head, complete with bird. How this squares away with the primary cast being anthropomorphic ducks is never explained.
- Ironic Name: The name of the captain of the Outer Space Patrol, charged with preventing Wacko from using his explosive seismospheres to wreak intergalactic havoc? Big Nasty. Small wonder Gosalyn thought he was the bad guy and Wacko was the good guy.
- Noodle Incident: When Drake and Gosalyn return home with Wacko, they find Launchpad with a birdcage on his head (complete with bird), a tutu around his waist, and a bowling ball in his hand. He is halfway through explaining how he ended up in this state to a bored Drake when Wacko interrupts them while fleeing a blowtorch-wielding Gosalyn.
- Restraining Bolt: Wacko's collar reduces him to half his normal size and less than half his normal strength, and prevents him from speaking. As soon as he meets the Mallards and the Muddlefoots, he indicates that he would very much like the collar removed, but doesn't approve of Gosalyn trying to use a blowtorch and a chainsaw to remove it.
- Shout-Out: After being freed of his collar, Wacko introduces himself by shaping his arms around his head in the form of the circle-and-hole-around-a-character opening introductions of classic cartoons (including Disney's own animated shorts).