There are two parts to the intro, easiest way to play them at the same time is with two people

Intro part A
e|-----------------|
B|-----------------|
D|----------1-0----|
G|----2-3------3---|
A|-3---------------|
E|-----------------|
(repeat x4)

Intro part B
e|-8----0--1--4--8-9-8|
B|------------------------|
D|------------------------|
G|------------------------|
A|------------------------|
E|------------------------|
(Play twice after 2nd repetition of intro A on another guitar)

Verse 1:
	Fm						C
When criminals in this world appear and break the laws that they should fear
	C7					Fm		Bbm
And frighten all who see or hear the cry goes out for far and near for

Chorus:
Fm		Ebm			Fm		Ebm
Underdog	(underdog)		Underdog	(underdog)
Fm			Ab			Fm		Ab
Speed of lighting, roar of thunder, fighting all who rob or plunder
C			C7		Fm		Bbm   Fm
Underdog 	(ooh ooh ooh)	Underdog 	(underdog)

This chord progression is repeated throughout the whole song, except the ending 
which goes like this:

Fm			Ab		Fm		Ab
Now unleash the dog of wonder, tearing evil souls asunder
C		C7			C#					C#7
Underdog	(bow-wow)		Oh what a wonder-dog	(and how)
			C	C7		Fm		Bbm7	Fm
Now here’s the epilogue	(ooh)		Underdog	(underdog)