Purple Tentacle, one of the crazy Dr. Fred's creations, drinks contaminated water from Dr. Fred's Sludge-o-matic. He mutates into an insane genius and grows arms, and now he's plotting to take over the world. In an effort to stop him, Dr. Fred sends three friends, Bernard, Hoagie and Laverne back in time to yesterday, in order for them to turn off the Sludge-o-matic and stop the sludge from spilling into the river, thus preventing the whole incident. Naturally, the cheap doctor uses a fake diamond over a real one in his time machine, which blows up sending Hoagie 200 years into the past and Laverne 200 years into the future.
Now the player must bring back Hoagie and Laverne, with the help of the time machine's ability to move objects between time periods, and then stop evil Purple from taking over the world.
Day of the Tentacle is a point-and-click adventure game and a sequel to Maniac Mansion. The player controls Bernard, and later also Hoagie and Laverne, being able to switch between them at any time. By using the commands at the bottom of the screen, the character can pick up items, use them on other things, talk to people and more. Any of the three friends can also send his items across time to another friend.
The game takes place in the same area, but in three different eras, and thus affecting history takes a vital part in some puzzles. For instance, Hoagie can hide an item in the 18th century, and Laverne discovers it in the 23rd century - but by then, the item may have been affected by time and changed its properties.
The CD version of the game contains voice-overs for all the dialogues.