Cherubim and Seraphim might be my favorite Morse show. I'll never know because its clear that there were three parts of material, but the powers that be slashed it down to two episodes, That led to an unusually poor conclusion and numerous loose ends. Does a "director's cut" exist?
The plot takes a while to develop, but once it does it bifurcates into a combination of pharmaceutical experimentation and medical ethics, and the drama of teenagers exposed to a new form of Ecstasy meant for seniors. It turns out be a great high but have very depressing after-affects on teenagers.
Morse's niece commits suicide very unexpectedly, as does another teen. Morse learns about the Rave scene and tries to save a girlfriend of his niece who is the thrall of the drug.
The twin plot lines never meet. Morse vows to find his niece's "murderer" and seems to suspect the scientist. But the third part of the show was never made.