Sonic & Knuckles arrived in the form of a very clever lock-on format cartridge, so that SEGA could continue their massive Sonic 3 saga. To get the most out of S&K, you of course need Sonic 3 but if you don't have it, SEGA has made it compatible with Sonic 1 and Sonic 2 as well. But on its own, S&K makes for a decent game too...
Launch Base has been destroyed by Sonic. Eggman has taken his plot to power the Death Egg to the next level. Meanwhile, Knuckles is a lazy git and decides to nap a bit, when a bomb explodes. Ouch. Knuckles survives the explosion and believes that this the work of Sonic. Chasing the disguised culprit, he plans to get to the bottom of it all. The ultimate showdown is about to commence!
Hooray. We finish the Death Egg saga with Sonic & Knuckles. Not much has changed since Sonic 3, apart from the fact that unfortunately, you can't play as Tails. Boo! There's also no Save File. And the game seems just a tiny bit shorter. None of this should worry us though, as when locked on with Sonic 3, the two combine to create a massive game. You start from the collapse of Launch Base Zone as you continue your adventure on good, solid floating ground.
Yeah, Robotnik's still here (you wouldn't think he... wouldn't be, would you?) and now he's really angry. Facing off with him every two acts, with a loyal robot slave at the end of Act 1 of each Zone, his anger shows with a new set of artillery. Spikes, fire, lava... you name it, Eggman's got it at his disposal! Chase him to the Death Egg for a Final (?) Showdown! Knuckles, meanwhile, has to contend with Eggman's robot version of himself and defeat a new robot Sonic clone, Mecha Sonic.
With Knuckles just a little bit peeved at the fact that some guy (i.e. 'Sonic') almost killed him with a bomb, its no surprise that he'll do whatever he can to stop the real Sonic from passing Angel Island the easy way. He still appears at points, making Sonic's life just a tiny bit less tolerable.
As you guessed, since Sonic & Knuckles was supposed to be with Sonic 3 in the first place, it's no wonder that SEGA have kept the Special Stages intact from Sonic 3. In order to collect the Chaos Emeralds you must find a giant ring located anywhere in a particular zone. When you find it, jump into it and you get taken to a 3D-type world where you have to collect blue spheres while avoiding red ones. Collect all the blue spheres in a stage and you collect the Chaos Emerald. Easy, no?
Of course, just playing as Sonic throughout the whole game and not with Tails is a bit boring now, seeings as Sonic Team have moved on from the classics of Sonic 1. But hold on! There is a new playable character to play through the game and his name is... Knuckles! Yes, at last, you can see how the Echidna's story unravels as you play through the levels.
Although they are the same levels as Sonic (except for the last few ones), Knuckles can access certain areas that Sonic usually cannot, because of his strength. He also has the ability to climb up walls when you double-jump against them, and better yet are his gliding capabilities when you double-jump in an open space.
My only qualm with this game is that, due to the fact that this is supposed to be the concluding part of Sonic 3, you're only going to get the most out of Sonic & Knuckles if you also own its prequel too. If you have Sonic 3 to lock-on to it, then getting Sonic & Knuckles should be at the very top of your priority list. However if you don't, knock off about three marks from the overall score below.