Once the camera has panned over to Danger Mouse and Penfold's starting position (back in the sewers), you will have to pick up Penfold and throw him across a gap to a platform (while watching out for drops of toxic waste, leaking from a pipe overhead); once you have done so, you will have to switch to Penfold, and throw a switch to open a trapdoor. You will then have to switch back to Danger Mouse, jump over to the first platform (while again watching out for those drops of toxic waste leaking from the pipe overhead), pick up Penfold, and throw him across another gap to smash through a stack of three crates and land on a second platform - and once you have done it you will have to switch back to Penfold, climb down a ladder and collect the first "DM" badge. You'll then have to climb back up the ladder, drop down to the vertically-moving platform and a third platform in turn, jump up to a fourth platform and collect the second "DM" badge; once you have done so, you will have to drop down through the (now-opened) trapdoor to land on a staircase, jump onto another vertically-moving platform and a fifth platform in turn and collect the third "DM" badge. You must now jump back onto the second vertically-moving platform, jump up to two more platforms in turn and collect the fourth "DM" badge - and once you've done so, you will have to retrace your steps back down to the staircase, switch back to Danger Mouse, walk back to the left and drop down through that now-open trapdoor to land on the staircase.
You will then have to pick up Penfold and throw him up to an eighth platform; once you've done so, you will have to jump up to the eighth platform, drop down to the ground, switch back to Penfold, follow Danger Mouse's path, switch back to Danger Mouse, pick up Penfold and throw him at second stack of three crates, to smash through them. You shall then have to walk up another staircase (while watching out for more drops of toxic waste leaking from second pipe overhead), switch back to Penfold, follow Danger Mouse's path, switch back to Danger Mouse, jump onto three more platforms in turn and jump back to the left to collect the fifth "DM" badge, landing on the tenth platform - and once you do this, you shall have to jump back up to the eleventh platform and a twelfth one in turn, throw another switch to cut off the flow of toxic waste, retrace your steps back to the top of the second staircase, pick up Penfold and throw him across a pool of toxic waste (which previously had more flowing into it). You will now have to jump back up to the ninth, tenth, eleventh, and twelfth platforms in turn, drop down to a thirteenth platform, switch back to Penfold, climb a second ladder, switch back to Danger Mouse, pick up Penfold and throw him across another gap to a fourteenth platform; once you have done so, you will have to switch back to Penfold, drop down to two more platforms and the ground in turn and walk onto a metal button to unlock the door to your left (but if you walk off it, it shall lock it again). You then have to switch back to Danger Mouse, climb down the second ladder, walk through the door and drop down onto another gray button to activate a third vertically-moving platform (but if you get off it, it will stop moving again) - and once you have done this, you shall have to jump onto the third (now-static) vertically-moving platform, a seventeenth platform, a fourth static vertically-moving platform, and an eighteenth platform in turn and throw a third switch to activate the fourth vertically-moving platform. You will then have to jump up to an eighteenth platform, walk to the left up a third staircase, and collect the sixth "DM" badge; once you have done so you will have to jump back onto the fourth vertically-moving platform and a nineteenth one in turn, jump over two more pools of toxic waste and collect the seventh "DM" badge. You shall then have to retrace your steps back down to the ground, drop down onto a third gray button, to activate a fifth vertically-moving platform (but for the second time, if you get off it, it will stop moving again) - and once you've done so, you will have to jump onto the fifth (now-static) vertically-moving platform and a twentieth platform in turn, and drop down onto a fourth gray button, to activate a sixth vertically-moving platform (but for the third time, if you get off it, it will stop moving again). You'll then have to jump up to the sixth (now-static) vertically-moving platform and two more platforms in turn, switch back to Penfold, jump alternately onto those third, fifth and sixth vertically-moving platforms and seventeenth, twentieth, twenty-first, and twenty-second ones in turn (if Danger Mouse didn't get off those gray buttons while the vertically-moving platforms were positioned high enough for Penfold to jump up to the static ones from, you'll have to switch back to him and retrace your steps back to them until they are), climb up a third ladder and throw a third switch to open second door below you (however, this will also open a third door behind you, and free an alligator, and you may remember from your first encounter in Episode 2 that they're too low to kill by a kick).
You then have to switch back to Danger Mouse, drop back down to the ground (unless he is already there), retrace your steps back to the left through the third door (jumping over the now-freed alligator if you have to) and collect the eighth "DM" badge; once you have done so, you will have to retrace your steps back to the right through the second door (again, jumping over the now-freed alligator if you have to), climb up a fourth ladder and the third one in turn, walk down a fourth staircase, kick a toxic waste monster three times to kill him, climb down a fifth ladder and collect the ninth "DM" badge. You will then have to climb back up the fifth ladder, walk down a fifth staircase, kick another toxic waste monster three times to kill him, switch back to Penfold, follow Danger Mouse's path (but you don't have to climb down the fifth ladder), switch back to Danger Mouse, pick up Penfold and throw him across a fourth pool of toxic waste to a twenty-third platform - and once you've done it, you'll have to jump over the fourth pool of toxic waste, jump up onto a twenty-fourth platform, and collect that tenth "DM" badge. You'll then have to climb up a sixth ladder, jump over another alligator onto a twenty-fifth platform, and walk to the left towards the exit; once you have done so, you will have to switch back to Penfold and follow Danger Mouse's path to the exit. The "MISSION COMPLETE!" text shall then be displayed upon the screen for an eighth time - and the duo will then proceed onto Episode 9.