While Lisa was building a new arm for Leni, something goes wrong and Leni ends as a water spirit and Lincoln as a fire spirit.
Remember the incident four episodes ago with Leni's arm? Well, after organizing her time, Lisa investigated how to recreate human flesh. Some days later, the arm was complete, only it missing attach it to the rest of the body. Leni is brought by Lincoln to Lisa's room to start the process. After removing her provisional arm, Lisa tells Lincoln to put into an indicator of super-strenght and slightly radioactive glue to fix the arm, in exactly 4 drops. Lincoln misheards and mistakenly puts 44. This makes Leni start a transformation into a water spirit. In her transformation, Lincoln tried to clean the excess of substance, which caused him to convert also in an spirit, in his case of fire.
When the transformation is done, both look very different. Leni discovers she can control the water of all the taps, which would help her with her showers (the water pressure it's not so good we say); while Lincoln can control the fire, something useful to cook because the gas always runs out and the flame's not so big. Both start enyojing their powers. Until both accidentally shoot at the same place at the same time, creating a small glitch. A vortex that suctions the water and fire of the planet. The vortex starts small, but when it aspires more, it gets bigger. And because they were made of that elements, Lincoln and Leni are suctionated into the vortex.
Lisa tells to the rest of the family about this incident. They are used to it because that happened like ten times in a year. Still, they decide to remediate it. The vortex is made from steam energy (that thing that forms when you do hot water 🥁). To extinguish the steam, we have to low the temperature. They localize the termostater, and adjust the temperature to -3º C. The vortex dissintegrates, and the water and fire return. But Lincoln and Leni haven't returned yet. The vortex led to a place near Bikini Bottom (where is common to see fire underwater), near Hawaii. The siblings had lost their powers, so they go up to the surface. Fortunately, they find a plane in an island, so they ride it and go back home. Oh, and if you ask me, yes, Leni recovered her arm.