This song is the opening track for the cd in Absentia for those f you who don't know. (why wouldn't you know?)
It's in Drop D tuning.

Rest for about 20 seconds in the beginning of the song.
Hard Riff
G-----------------------------------------------|------------|
D-----------------------------------------------|------------|
A-----------5~-4~--3~-35----------5~-4~--3~-----|------------|
D-320-320-30--0--0--0---320-320-30--0---0--0-3~-|-320-320-30-|

Hard riff con't
G---------------------------------------------------------|
D---------------------------------------------------------|
A----------530---------------530---------------530------5-|
D-0-3-5-6-0---53030-0-3-5-6-0---53030-0-3-5-6-0---53030---|

Verse
G---------------------------------------------|
D---------------------------------------------|
A-3--3--33-3-2--2--22-2-0--0--00-0-0--0--00-0-|
D---------------------------------------------|

G------------------------------------------------------------------------------------------------------------------------|-----------------|
D------------------------------------------------------------------------------------------------------------------------|-----------------|
A-5--5--55--5--5--55--7--7--77--7--7--77--5--5--55--5--5--55--3--3--33--3--3--33--5--5--55--5--5--55--7--7--77--7--7--77-|-3-3-33-2-2-22-0-|
D------------------------------------------------------------------------------------------------------------------------|-----------------|

G-----------------------------------------------|-------------------------|------------|
D-----------------------------------------------|-------------------------|------------|
A-----------5~-4~--3~-35----------5~-4~--3~-----|-------------55--5----55-|------------|
D-320-320-30--0---0--0--320-320-30--0---0--0-3~-|-320-320-30-0--00-00-0---|-0-3-5-7~-|

G-----------------------------------|
D-----------------------------------|
A-5~--7~--5~--3~--5~--7~--7/3-3~-2~-|
D-----------------------------------|

Finish the song with the opening riff.

That's it, this is a great Porcupine Tree song and it's really  cool to play.