Shampoo's Red Thread of Dread!

Shampoo's Red Thread of Dread! (Shanpū no Akai Ito シャンプーの赤い糸) is the 19th episode in the fourth season known as "Outta Control", it is the 83th episode overall.

Plot Summary
Ranma suddenly falls head over heels for Shampoo when she ties him to her with the magical Red Thread of Fate. Just a few more hours and the effect will be permanent, and Akane doesn't care one bit... does she?

Trivia
In East Asian mythology, the gods tie a red cord around the ankles of those that are to meet one another in a certain situation or help each other in a certain way. Often, in Japanese culture, it is thought to be tied around the little finger. The two people connected by the red thread are destined lovers, regardless of time, place, or circumstances. This magical cord may stretch or tangle, but never break. This myth is similar to the Western concept of soulmates or a destined flame.