It's fun, and the central buddyship makes it all worthwhile, but it really is the biggest load of old toot, as Mr Sugar might say. Looking back to the first three-part story, as far as I could figure out it went like...
A UK pharma company tests a new drug in India, but rather than using freely available locals for thirty quid and a bed for the night, it opts to recruit travellers at £600 a pop.
One of these five testees (or, presumably, many hundreds - even a dodgy pharma company wouldn't consider a five person clinical trial to be even worth carrying out, much less definitive) has a funny turn, due to his enthusiasm for recreational drugs.
Years later, the company, realising that one or more of the other five might have been a witness to his funny turn, and let the cat out of the bag, thereby imperiling their cash cow (which presumably has by now been used by hundreds of thousands of people who have unaccountably failed to go psycho) decides that rather than print a warning saying 'DO NOT TAKE IF FULL OF ILLICIT SUBSTANCES' the best thing to do would be to recruit The Sinister Sisters, have all the other triallists murdered, and pin the blame on the chap who had a funny turn.
And then when it all unravels, everyone immediately admits everything, some of them get snuffed by The Sinister Sisters (last seen riding off into the sunset) and all the loose ends are wrapped up neatly in, ooh about an hour and a half.
The End.
I mean, it's still fun, but it really does test new limits in tv drama implausibility. If not incoherence.