My favorite Sonic game one of my favorite games of all time.
The story is broken into 6 sections, so here is a summary for Big, and Gamma since they have the least to do with the overall story. Big lost his frog and has to fish to get him back and E-102 Gamma is a robot created by Dr. Eggman (I call him Eggman because I grew up with Sonic X so shut up) who turns on him and decides to destroy all of the E series robots to free the birds captured inside and then kills himself to free his bird. Sonic's story is that one day he comes across a water monster called Chaos and finds out Dr. Eggman is working with the monster and says that feeding the 7 chaos emeralds will turn Chaos into Perfect Chaos so he can destroy Station Square to build Eggman land. Sonic and Tails try to collect the chaos emeralds, but Eggman keeps on taking the emeralds from them. Eggman then unleashed his most powerful creation, the Egg Carrier, which is where Sonic beats Chaos for the last time and then chases Eggman down to the Mystic Ruins to have the final showdown. Tails story is the exact same except the ending where Tails saves Station Square from Eggman. Knuckles is trying to collect all of the pieces of the Master Emerald after Eggman destroys it to release Chaos. Knuckles' story ends on the Egg Carrier where he does the finishing job on Chaos and takes the 6 emeralds back to the emerald shrine. Amy is protecting a bird from a robot known as Zero hunting the bird because it has a chaos emerald with it. At the end of her story she beats up Zero for attacking the bird. Lets also talk about Tikal shall we? Tikal is a ball of light that helps you throughout the game who used to be an echidna that sealed herself and Chaos inside the Master Emerald to protect the world from Chaos' wrath. In the last story segment Chaos betrays Eggman and steals the 6 emeralds from Knuckles and takes the emerald Sonic and Tails are using to power their plane. Chaos then becomes Perfect Chaos and floods Station Square then Sonic uses the chaos emeralds to turn into super Sonic and defeats Chaos. Chaos then goes with Tikal into the afterlife.
Now lets talk about the gameplay. I will say now Sonic controls the best in this game compared to all of his other 3D games, Sonic can do the spin dash, jump on enemies, use the light speed dash after getting an upgrade, and use the homing attack which is done by pressing the jump button twice and Sonic homes in on the closest enemy. Tails controls the same as Sonic except that by pressing the action button Tails does the tails attack and doing this while running will let him roll into a ball and of course by pressing the jump button while in midair Tails can fly. Tails' levels are also always a race. Knuckles can punch enemies by pressing the action button and while running pressing that button will have Knuckles roll into a ball. Jumping and pressing the jump button will let Knuckles glide and can climb a wall with his fists. Knuckles is hunting for 3 emerald pieces every level and whenever you are near any piece the radar will let you know how close you are to that piece (unlike Sonic Adventure 2). Amy's method of attack is using her hammer. Amy is slower than the others, but when at top speed Amy does a super jump by using her hammer attack. Amy's levels are more like a Mario game except that Zero is constantly chasing her in the level. Gamma's levels are shooting levels while under a time limit, but you can extend you time by shooting more things. Gamma can also hover after getting an upgrade. Big just fishes.
Overall Sonic Adventure is a solid game that any fan of Sonic or platformers should try out.