The game uses most of the characters from the original Maniac Mansion game, some of which themselves were taken from the TV series of the same name. The story is an intricate one, involving Dr. Fred Edison, mad scientist extraordinaire, and his two intelligent tentacle creations, Purple Tentacle and Green Tentacle, which first appeared in Maniac Mansion. Purple Tentacle drinks the large quantities of toxic waste which pour from Dr. Fred's lab, turning him into an evil genius bent on world domination (what else?). Bernard Bernoulli, one of the heroes of the game, manages to set Purple Tentacle loose, and Dr. Fred attempts to send him and his friends Hoagie and Laverne back in time in order to stop Purple Tentacle 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 has to find a non-imitation diamond 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 utilises the three time streams effectively, with several puzzles depending on actions in one era affecting the other eras.
The game was released simultaneously on floppy disk and CD-ROM, with the CD-ROM version featuring a full voiceover soundtrack. It was designed by Dave Grossman and Tim Schafer.
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 (on the Mac, the computer needs to be used five times in a row).