According to Ian Rankin the Rebus series stems from the author's desire to use the character to explore issues he wanted to write about rather than an interest in crime fiction as a genre and in Mortal Causes he's found an interesting sociopolitical issue to play around with.
Against the backdrop of the Edinburgh Festival this exploration of links between Scottish nationalism and Northern Ireland Loyalist paramilitaries might seem to be stretching things considerably, since you'd expect the Republican side off the Ulster Troubles to be a more likely fit with Scottish separatists, but if you can suspend that notion for a bit what happens here works fairly logically, though the plot line develops with the regulation quota of twists, turns, blind alleys and dead ends.
It's fairly obvious from the start that there's some Ulster influence in the murder in Mary King's Close that starts things off, with the IRA known to render enemies or traitors immobile for the rest of their lives with bullets to the elbows, kneecaps, and ankles. Having done that, however, in this case a seventh bullet has finished the victim off.
Things become complicated fairly quickly when the victim is identified as the previously unknown son of gangster 'Big Ger' Cafferty who might be behind bars but certainly isn't going to pass up any opportunity to exact revenge. An SaS tattoo seemingly links the body to Sword and Shield, a supposedly defunct Scottish Nationalist group and Rebus finds himself seconded to the SCS (Scottish Crime Squad) working with London-based Special Branch DI Abernathy, a scenario which coincides nicely with Rebus' tendency to follow his nose regardless of instructions from above.
That tendency is aided by his SCS colleagues' reluctance to let the newcomer in on the action, and in the end the whole secondment is a fairly adept bit of manipulation in the course of another, related, investigation.
There are also the usual related subplots along the way. The Rebus/Patience relationship continues across thorny territory, with its progress not exactly helped by a rash encounter with a randy lawyer with ambitions of her own.
Rebus' continuing contact with the Catholic priest he ran across in The Black Book throws in an additional subplot, involving a youth club at Pilmuir's Garibaldi Estate, which seems at first to be tangential but eventually delivers Rebus to the heart of the matter.
In the end, though, the story represents a fairly chilling exploration of the ways in which seemingly incompatible entities can cut deals with each other, and when Big Ger engineers an escape from jail to exact his own version of revenge he's also providing the fuel for Rebus continuing obsession with bringing him down.