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. This was the last SCUMM game to use the original interface of having the bottom of the screen being taken up by a verb selection and inventory; starting with the next game to use theSCUMM engine, Sam and Max Hit the Road, the engine was modified to scroll through a more concise list of verbs with the right mouse button and having the inventory on a separate screen. This formula carried on to later games in the franchise, such as The Dig, Full Throttle and The Curse of Monkey Island.
Day of the Tentacle uses time travel extensively; early in the game, the three main protagonists are separated across time by the effects of a faulty time machine. The player, after completing certain puzzles, can then freely switch between these characters, interacting with the game's world in the separate time periods. Certain small inventory items can be shared by placing the item into the "Chron-o-Johns", modified portable toilets which instantly transports them to the other time period, while other items are shared by simply leaving the item in a past time period to be picked up by a character in a future period. Changes made to a past time period will affect a future one, and many of the game's puzzles are based on the effect of time travel, aging of certain items, and alterations of the time stream. For example, one puzzle requires the player, while in the future era where Purple Tentacle has succeeded, to send a medical chart of a Tentacle back to the past, having it used as the design of the American flag, then collecting one such flag in the present and sending it to the future to be used as a Tentacle disguise to allow that character to roam freely.
In Maniac Mansion, the playable characters can be killed by various sequences of events. LucasArts adopted a different philosophy towards its adventure games in 1990, beginning with Loom. 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 Loom, including Day of the Tentacle, the player character(s) cannot die.
The whole original Maniac Mansion game can be played on a computer inside the Day of the Tentacle game; this practice has since been repeated by other game developers, but at the time of Day of the Tentacle's release, it was unprecedented.
The game, which takes place five years after Maniac Mansion, opens with Purple Tentacle becoming exposed to toxic waste spewing from Dr. Fred Edison's mansion. The toxic waste isn't the by-product of any experiment, it's a device designed specifically to pump toxic sludge out of the building, as it is all the rage with mad scientists lately. Purple's ingestion of it results in him growing a pair of flipper-like arms, vastly increased intelligence, and acquiring a thirst for global domination. Dr. Fred catches Purple Tentacle, as well as the friendly, non-evil Green Tentacle, and keeps them both in his basement before deciding that he will euthanize them. That evening, Green Tentacle sends a plea of help to his old friend Bernard Bernoulli — a stereotypical nerd and character from the first game — who heads off to the mansion to rescue him, accompanied by his friends, Laverne, a slightly psychotic medical student, and Hoagie, a laid-back heavy metal roadie. Bernard frees Green and Purple, only for Purple to inform him of his plans of world domination and resume his conquering of the Earth. Since he can't figure out a way to stop Purple now that he's begun his mad quest, Dr. Edison attempts to send the three friends back in time to 'yesterday' using his time machine, which consists of a central unit made out of an old car and three personal travel units called "Chron-o-Johns", made from portable toilets. By doing so, they can turn off the sludge machine which produced the toxic waste so that Purple never ingests the waste in the first place, hence pre-emptively stopping him from taking over the world. However, because Dr. Edison has used an imitation diamond as the power source, the machine fails, sending Hoagie 200 years in the past, where he learns that the mansion was the setting for the creation of the United States Constitution, and Laverne 200 years in the future to a Tentacle-controlled world where humans are treated as pets and Purple is revered as a hero, while Bernard remains in the present. Dr. Edison tasks Bernard with finding a real diamond to power the time machine to return his friends to the present so that they can try to travel to yesterday once again, while informing Hoagie and Laverne through the Chron-o-John that they must find power sources for their own units in order to bring them back. Fortunately for all involved (especially Hoagie, who has been sent back to before electricity was available), the three can send small objects back and forth in time through the Chron-o-Johns in order to complete these tasks.
Eventually, Bernard uses Dr. Edison's old family fortune to buy a real diamond, both Laverne and Hoagie manage to power their units, and the three are reunited in the present. Dr. Edison attempts to send them back into the past again, this time successfully. Upon arrival, they find that the Purple Tentacle from 200 years in the future has also used the time machine to bring several versions of himself to the same day to prevent them from turning off the sludge machine. Bernard and his friends successfully defeat all the Purple Tentacles, turn off the machine and restore the course of future events to normal order. The game ends with the credits rolling over a Tentacle-shaped American flag, one of the more significant results of their tampering in history.
One of the aspects of Day of the Tentacle's plot is that it gives the game player the opportunity to interact with the comedic, cartoony versions of several figures from colonial America, like George Washington, Thomas Jefferson, Benjamin Franklin, John Hancock and Betsy Ross; whose descendants (or at least characters that resemble them) can be spotted in the other ages. Harold, seemingly a descendant of Washington, appears as a transvestite in a future beauty contest organized by the Tentacles. An apparent descendant of Ben Franklin makes an appearance as a novelty toy salesman and a descendant of John Hancock appears as a depressed inventor named Dwayne.
Some of the more entertaining puzzles of the game involve these characters. In one sequence, Hoagie must give an exploding cigar to Washington in order to replace his famous false teeth with chattering novelty mechanical dentures, while in another he gives a drawing of a tentacle to Ross, who sews it into the American flag. In another scene, in order to coax Washington into chopping down a kumquat tree, Hoagie must paint the fruits red, as Washington insists that he only chops down cherry trees, referring to a legend concerning Washington's youth.
Day of the Tentacle was well received at the time of its release, and still features regularly in lists of 'top' games. Adventure Gamers included the game as #1 on their 20 Greatest Adventure Games of All Time List.IGN rated it number 60 on their 2005 top 100 games listand CVG has it at number 30, it is also listed as one of the greatest games of all time on GameSpot.
Adventure Gamers' review rated the game 5 out of 5, stating "If someone were to ask for a few examples of games that exemplify the best of the graphic adventure genre, Day of the Tentacle would certainly be near the top".
The game was reviewed in 1993 in Dragon #199 by Sandy Petersen in the "Eye of the Monitor" column. Petersen gave the game 4 out of 5 stars.
The game was rated 93 out of 100 on Metacritic.