Five years after the events of Maniac Mansion, Purple Tentacle—a mutant monster and lab assistant created by mad scientist Dr. Fred Edison—drinks toxic sludge from a river behind Dr. Fred's laboratory. The sludge causes him to grow a pair of flipper-like arms, develop vastly increased intelligence and a thirst for global domination. Dr. Fred plans to resolve the issue by killing Purple Tentacle and his harmless, friendly brother Green Tentacle, but Green Tentacle sends a plea of help to his old friend, the nerd Bernard Bernoulli. Bernard travels to the Edison family motel with his two housemates, deranged medical student Laverne and roadie Hoagie, and frees the tentacles. Purple Tentacle escapes to resume his quest to take over the world.
Since Purple Tentacle's plans are flawless and unstoppable, Dr. Fred decides to use his Chron-o-John time machines to send Bernard, Laverne and Hoagie to the day before to turn off his Sludge-o-Matic machine, thereby preventing Purple Tentacle's exposure to the sludge. However, because Dr. Fred used an imitation diamond rather than an real diamond as a power source for the time machine, the Chron-o-Johns breaks down in operation. Laverne is sent 200 years in the future, where humanity has been enslaved and Purple Tentacle rules the world from the Edison mansion, while Hoagie is dropped 200 years in the past, where the motel is being used by the Founding Fathers as a retreat to write the United States Constitution. Bernard is returned to the present. To salvage Dr. Fred's plan, Bernard must acquire a replacement diamond for the time machine, while Hoagie and Laverne must restore power to their respective Chron-o-John pods by plugging them in. To overcome the lack of electricity in the past, Hoagie recruits the help of Benjamin Franklin and Dr. Fred's ancestor, Red Edison, to build a superbattery to power his pod, while Laverne evades capture by the tentacles long enough to run an extension cord to her unit. The three send small objects back and forth in time through the Chron-o-Johns and make changes to history to help the others complete their tasks.
Eventually, Bernard uses Dr. Fred's family fortune of royalties from Maniac Mansion to purchase a real diamond, both Laverne and Hoagie manage to power their Chron-o-Johns, and the three are reunited in the present. Purple Tentacle arrives, hijacks a Chron-o-John and takes it to the previous day to prevent them from turning off the sludge machine; he is pursued by Green Tentacle in another pod. With only one Chron-o-John pod left, Bernard, Hoagie and Laverne use it to pursue the tentacles to the previous day, while Dr. Fred uselessly tries to warn them of using the pod together, referencing the film The Fly. Upon arriving, the trio exit the pod only to discover that they have been turned into a three-headed monster, their bodies merging into one during the transfer. Meanwhile, Purple Tentacle has used the time machine to bring countless of versions of himself from different moments in time to the same day to prevent the Sludge-o-Matic from being deactivated. Bernard and his friends defeat the Purple Tentacles guarding the Sludge-o-Matic, turn off the machine and prevent the whole series of events from ever happening. Returning to the present, Dr. Fred discovers that the three have not been turned into a monster at all but have just gotten stuck in the same set of clothes; they are then ordered by Dr. Fred to get out of his house. The game ends with the credits rolling over a tentacle-shaped American flag, one of the more significant results of their tampering in history.By: binford6001