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Day of the Tentacle begins some time after the events in Maniac Mansion. Of the six playable characters in the first game, only Bernard Bernoulli, a stereotypical nerd character, makes a return appearance. This time, Bernard has a starring role, as he leads his two new friends, Hoagie and Laverne, back to the Mansion. One of Dr. Fred Edison's two creations, the Purple Tentacle, drinks radioactive waste from the Mansion, growing two stubby appendages for arms and an appetite for global domination. Bernard and friends plan to travel back in time, using Dr. Fred's Port-a-Potty-turned-time machine (dubbed "Chron-o-John"), to stop Purple from drinking the toxic waste in the first place.
Unfortunately, the cheap imitation diamond in Dr. Fred's time machine fails, leaving Hoagie 200 years in the past, Laverne 200 years in the future (a future dominated by tentacles) and Bernard still in the present. Bernard, with the assistance of budding rock star Green Tentacle, has to find a non-imitation diamond in order to make the time machine functional once more, and Hoagie and Laverne both have to get back to their time machines and connect them to a power source, in order to team up again and finally defeat Purple Tentacle. The game utilizes the three time streams effectively, with several puzzles depending on actions in one era affecting the other eras.
One of the most notable features of Day of the Tentacle was that it featured the entire original Maniac Mansion built into the game. To play it, simply use Weird Ed's computer (in the Macintosh version of the game, the computer needs to be used five times in a row).
Day of the Tentacle follows the point and click two-dimensional adventure game formula, first established by the original Maniac Mansion. Players direct the controllable characters around the game world by clicking with the computer mouse. To interact with the game world, players choose from a set of commands arrayed on the screen and then on an object in the world. In Day of the Tentacle, the player can switch between of any one of the three playable characters at any time. The three protagonists can also share inventory items amongst themselves, a feature that plays into many of the game's puzzles.
In Maniac Mansion, the playable characters can be killed by various sequences of events. LucasArts adopted a different philosophy towards its adventure games in 1991, beginning with The Secret of Monkey Island. Their philosophy was that the game should not punish the player for exploring the game world. Accordingly, in most of the adventure games released by LucasArts after The Secret of Monkey Island (including Day of the Tentacle), the player character(s) cannot die (there is one exceptional sequence in which the player character of Monkey Island could perish; see that game's article for more information).
There have been a few fan groups in Germany who have started developing fan-games, but as of 2005, none of them are complete. As of this time LucasArts seems to be very unreceptive to the idea of making any more adventure games or allowing others to commercially develop their old licenses.