A Molecular Destabilizer (also known as a Wibbly Gun by the Cat or the Moly-D by Kryten) is a hand-held weapon which has the capability to temporarily destabilize the molecular structure of solid objects, usually to allow other objects or individuals to pass through otherwise solid walls and floors.
It is unknown where the gun came from, but three million years into the future it was the favoured tool of the rogue droid and thief Hogey the Roguey, who used the Moly-D to gain access to various spaceships. The gun would eventually be used by the boys from the Dwarf as part of a plan by Rimmer to defeat the Simulant Generals.
A rogue droid called Hogey the Roguey uses a Moly-D to gain entry to the G Deck of Red Dwarf from his ship, having previously used the weapon to board a Simulant Death Ship to steal a a prized cloth map of the galaxy from them. Hogey had a strange obsession with the Dwarfers, often challenging them to "duels across time and space", and had apparently stolen the galactic map to use as leverage in these games.
When the Simulant Generals finally track the map down, they fire Photon Mutilators on the mining ship Red Dwarf, blasting a hole in the bulkhead. This causes Lister's bunk room to de-pressurise as air is sucked out into the vacuum of space. Ultimately, this also causes Hogey himself to be sucked towards the crack, sealing it. While Hogey is held in place by the suction, Lister steals his Molecular Destabilizer, and makes his escape.
While making their way to a Blue Midget craft, Lister test-fires the weapon towards the floor, which unexpectedly causes him to fall straight through to the deck below. He realises what happened, and immediately fires at the ceiling to bring Rimmer, the Cat and Kryten down to him. The Cat first explains it as a "wibbly gun", which makes things go all "wibblified". Kryten then informs them of its actual power, but Rimmer still insists that the Cat's explanation was clearer.
Eventually, while surrounded on all sides by the Simulant Death Ship and several Annihilators, Lister uses the Molecular Destabilizer on Blue Midget at the last second to destabilize all four of its bulkheads, causing the inbound missiles launched from the Simulant ships to pass straight through Blue Midget as if the craft was not there. In tune with Rimmer's battle-plan, their alignment allows for the missiles to head straight towards the Simulants' own ships, destroying them.
In the episode's post-credits sequence, Hogey is shown to have somehow escaped (or it is perhaps actually "his brother"), and again brandishes another Molecular Destabilizer at the crew in his never ending quest to convince them to engage in an entertaining "duel across time and space" with him. ("The Beginning", Series X)