Jaq Draco is an Ordo Malleus Inquisitor, who has become a member of the inner circle (those with access to secrets and hidden twists).
Grimm is a squat, grunty, always complaining, but a good companion and mechanic.
Meh'lindi is a beautiful Callidus assasin, who has been chosen for a special treatment. Instead of using pholymorphine to just mutate into other humanoid forms, terrible surgery is done inside her to introduce special muscles and bones so that she can morph into a genestealer hybrid and infiltrate their cults.
Googol is an imperial navigator who likes poetry (and women), "driver" of Jaq in the warp.
Those four widely different characters, plus later on Lex, an Imperial Fists space marine captain, will start perfoming some imperial missions like killing a genestealer patriarch, but things will get interesting when Jaq finds about a über-secret plot with an unknown secret organization, regarding humanity's Emperor, Eldar, Chaos... And that's only the beginning.
I won't spoil anything because the beauty of this books is reading them without summaries/intros. Just believe me that if you could only read one Warhammer 40.000 book, it should be this one.
Ian Watson had lots of problems with the books because he chose to ignore Warhammer's main target (young people) and instead write adult, serious sci-fi novels. You have topics like sex, raping (mental, but anyway), violence, sacrifices... Not for children, but neither targeted for them.
The story unfolds so great and has quite a few good twists and revelations that it sucks you into reading more and more.
The best book I've read so far.