Plex was an old friend of the Doctor.

The Ninth Doctor visited him to deliver the news that his home planet had been destroyed in a freak singularity. The Doctor discovered that Plex had created hundreds of clones of himself to populate the empty planet where he resided. In light of Plex having recently become the last of his species, the Doctor realised that Plex's plan made sense and gave him a modified Chameleon Arch biodata module to "copy and paste" his memories into the clones.

A statue depicting the Ninth Doctor with the body of Plex's species was mounted over the entrance of Plex's tomb. (COMICThe Promise)

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