The Murder Game

It's only the second PDA, and the BBC have made a recurring monster in the form of the Selachians (the Waro should make a second appearance though)
The Murder Game starts out as a murder mystery inside a fictional murder mystery on the Hotel Galaxian, something which reminds me of that one Rick and Morty episode featuring Zigerian scammers, however, a bomb goes off (literally), causing the Hotel Galaxian to de-stabilise and start falling towards earth (it's a space station), and then the Selachians arrive.
The Selachians are one of the only PDA original monsters to get a second appearance (mentions in VMAs don't count), and for good reason because they aren't actually sharks, they actually are fish people in mecha shark suits, but you wouldn't exactly think about it with how action-packed it is, so, what are the Selachians after?  The ultimate weapon, basically it's Total Recall's asteroid in two CDs (god i love outdated Sci-Fi), and in the very electronicallized future of 2146 (according to the cover, though it apparently happens in 2136) this would very well indeed be an ultimate weapon.
Props to Steve Lyons, i actually had a messenger conversation with him, apparently, it's a five-part adventure with cliffhangers every three chapters(don't know if this is common knowledge or not, doubt it is though) 9/10