Gun in four parts

The gun in four parts was a gun which was pursued by Martha Jones during the Year That Never Was.

Characteristics
Ostensibly created to permanently kill a Time Lord while preventing them from regenerating, the gun worked using four chemicals which were slotted into it. (TV: Last of the Time Lords)

History
According to Martha, the gun was developed by "UNIT and Torchwood" as an "ultimate defense", having studied Time Lords in secret due to the Doctor and the Master's frequent visits to Earth.

During the Year That Never Was, Martha, having acquired the gun, supposedly travelled the Earth to collect the chemicals needed to use it, the first three of which she found in San Diego, Beijing and Budapest. The fourth was found at an old UNIT base in north London. Martha told this to Alison Docherty, who made aware of this in exchange for information regarding her misson son. Shortly after, Martha was intercepted by, who used his laser screwdriver to destroy the bag which contained the gun.

Aboard the Valiant, however, Martha revealed to the Master that the gun was but a ploy concocted by her and the Tenth Doctor; Martha had infact travelled the world to tell people of the Doctor, and had deliberately informed Docherty of the gun so that she would end up confronting the Master aboard the Valiant. The combined thoughts of humanity, amplified by the Archangel Network, allowed the Doctor to stop the Master and avert the Year That Never Was. (TV: Last of the Time Lords)