Traitor's Gambit (Ciaphas Cain)

On a tour of inspection on an aquatic world, in a sector uncomfortably close to Tau space, Cain and Jurgen take full advantage of the Governor's hospitality and conduct most of their tour from his luxurious submersible yacht - until a group of pro-Tau terrorists strike.

A short story from the 'Defender of the Imperium' omnibus.