Former Addicks loanee Darren Ward had set the Lions on their way with a first-half header before a manic five minutes in the second half turned it into a drubbing as Christian Dailly's own goal and Morison's double completed the rout.
After a scrappy opening to the game the deadlock was finally broken two minutes before the break. Charlton failed to defend a routine Chris Hackett corner, with Ward rising highest to power in a free header.
Hackett turned provider once more in the 74th minute as he cut in past full-back Chris Solly and put in a wicked delivery that Dailly could only turn into his own net.
It was three just a minute later as substitute Neil Harris sent Morison clear and he took the ball down and calmly drilled the ball past the helpless Rob Elliot.
And the Den faithful only had to wait another five minutes for the next goal. Hackett's drive was well saved by Elliot, but the ball fell straight to Morison who fired in for his second.