Three shapeshifting aliens from the planet Fleeblebrox are returning home after a holiday on Earth. But one of the aliens, Dtz, decides he likes it on Earth and, at the first opportunity, heads back there in a spacepod.
Meanwhile, at Rescue Ranger Headquarters, the gang find that Dale has raided the stash of walnuts Monty needs to make a batch of cookies. Dale tries to blame aliens, but Chip isn't fooled for a second and orders Dale to go and gather more walnuts. As it happens, Dtz is nearby and, when he sees Dale approaching, he changes his spacepod into a walnut - a very large one.
Dale, seeing the large "walnut", dumps his sack of regular-sized walnuts and lugs the former home. There, he discovers that the "walnut" is really a spacepod and meets Dtz. Excited at seeing an alien up close, Dale asks Dtz if he is going to take over the planet. But Dtz says he just wants to have fun and Dale suggests that the alien become one of the Rangers. Taking his cue, Dtz transforms into an exact copy of Dale, just as Chip comes to ask about the walnuts.
When Chip comes in, he sees Dtz (who is still disguised as Dale; the real Dale is hiding behind the door) and tells him Monty is ready for the walnuts. Dtz, of course, doesn't know about the walnuts and, as an exasperated Chip leaves, Dale realises having a double of himself could come in useful. The first thing Dtz and Dale do together is (with a little help from Dtz's telekinetic powers) gather enough walnuts for a huge batch of cookies.
For a while, Dtz continues to take care of Dale's share of the chores. But his companions, Bric and Brac, soon track him down and order him to return to Fleeblebrox. When, minutes later, he overhears the other Rangers discussing how much they prefer the "new Dale", Dtz has an idea. He tells Dale he is going to let him take his spacepod for a spin, then jams a back-to-front space helmet onto the chipmunk's head and sends the spacepod on its way on automatic pilot. But, as the spacepod zooms away, Dtz realises he has forgotten to unpack his stash of erfburgles, a favourite snack of his.
Dale soon finds himself on the Fleeblebroxian mothership and meets Bric and Brac, who think he is the real Dtz. Bric tells Dale to set course for the planet Umchuck 5 and Dale, excited at being left in charge of a spaceship's controls, presses several buttons at random and sends the mothership zooming across space. Meanwhile, Dtz (still posing as Dale) makes some adjustments to the Ranger Plane, enabling it to zoom round the world in a few seconds.
Later, Dale, who has been sent to Dtz's quarters after the incident with the mothership's flight controls, stumbles across the artificial gravity device. First, he adjusts the gravity so that it becomes twelve times stronger than on Fleeblebrox. Then, as an annoyed Bric and Brac drag themselves along the floor, he switches the gravity off altogether. But, just as he is beginning to enjoy being weightless, Bric and Brac turn the gravity back on and he falls to the floor.
Bric and Brac then start to fear that Dale (whom they, of course, believe to be Dtz) is "becoming an Earthling" and decide to take him back to Fleeblebrox for treatment. They start chasing Dale all over the mothership, armed with freeze-guns, which they plan to use to freeze him. But Dale escapes in a spacepod and heads back to Earth.
Meanwhile, the other Rangers are trying to get Dtz to relax, unaware that he is not the real Dale. Gadget berates Monty and Chip for wanting to change Dale and then not being happy when he does change. However, when she glances behind her and notices Dtz sweeping the floor without touching the broom, she admits that something strange is going on and persuades Dtz to join the others in watching a sci-fi movie.
Dtz decides to go and make some popcorn and walks past the window just in time to see Dale coming. He runs back to the others and tells them that shapeshifting aliens are attacking. But the others think this is just another example of Dale's habit of over-reacting to movies and Gadget tries to calm Dtz down. Just then, however, Dale bursts in and yells the word "aliens!", prompting the others to run for cover.
Dtz and Dale end up brawling on the floor. Monty pulls them apart, but the Rangers find it impossible to tell which one is which and each of them claims that the other is the alien. Then, as one of the pair pins the other to the floor, everyone is alerted by a rumbling sound coming from outside.
Rushing outside, the Rangers and Dtz see the mothership landing nearby as Bric and Brac emerge from it. Brac orders Dtz to surrender before they blow up the Earth, but Dtz thinks they are bluffing. However, when Gadget tells Bric and Brac that they will blow up too, they only laugh at her and reveal that, if they blow up, they can reassemble themselves immediately.
Dale quickly runs onto the mothership and returns with a tray of erfburgles. On seeing his favourite snack, Dtz immediately changes back into his usual form and leaves with Bric and Brac. The other Rangers are pleased to have the old Dale back.