Astronomers have argued for a distant, massive, eccentric and inclined planet to explain elongated and apparently clustered orbits of extreme Trans-Neptunian Objects. The so-called Planet 9 Hypothesis has  generated a flurry of both theoretical and observational works: most constraining Planet 9 properties and its location on the sky, some proposing alternative clustering mechanisms, and others dismissing the signal as observational bias, thus obviating the need for a shepherding mechanism in the first place.

I shall start this overview of the Planet 9 Hypothesis by highlighting the stunning precedent of the prediction and detection of Neptune, then proceed to a blow by blow account of the evidence for and current predicament of Planet 9. I will then briefly review an alternative we proposed, and conclude with recent work which provides a clean scaffolding for Planet 9 dynamics, as it points to applications in hierarchical systems, exo-planetary debris disks included. The whole will make for a leisurely guided tour of the Trans-Neptunian realm and the beautiful bits of celestial mechanics that were ushered by a most seductive idea, whose time is yet to come (if at all).