This is an exciting and informative through the history of 3D from its early beginings to the present day (including its use in Universal Studio's T2 ride in LA). Its set in a 3D lab of some proffesor of 3D, who orders his robot pal to inform (and most importnatly entertain) the audience with 3D pieces from over the years. You see early - but incredibley effective - stereoscopic pictures, corny 50s 3D movies with horrendous giant spiders and creaky wooden actors continuoisly throwing objects at the camera (to fully milk that 3D effect). It then comes up to date with some computer animations and so-real-it's-unreal filmed footage of various beaches and street scenes. The linking pieces of the proffessor in the lab are humourous and not in the slightest bit annoying (as is the case with the linking segmnets in many other imax movies - see e.g. my review of cyberworld).
As a 3D imax experince Encounters is also the best i've ever seen (and i've seen many - i've even been to an imax theme park in France "Futureworld"). As every image in the film is designed to make the most of the format, you really get that in your face sensation and will continuously try and grab at the air to feel for 3D objects that of course are not really there. In essence the audience really feels like they are watching a live show with actors on a stage rather than merely a series projected images. Imax movies in general are a mixed bag, some are really aweful. Here though is a true gem that makes the most of the format. You should definetley check it out if its playing anywhere near you.