Barkley, Shut Up and Jam: Gaiden
(note: the official full title is "Barkley, Shut Up and Jam: Gaiden, Chapter 1 of the Hoopz Barkley SaGa")
From author's description: "The Great B-Ball Purge of 2041, a day so painful to some that it is referred to only as the "B-Ballnacht". Thousands upon thousands of the world's greatest ballers were massacred in a swath of violence and sports bigotry as the game was outlawed worldwide. The reason: the Chaos Dunk, a jam so powerful its mere existence threatens the balance of chaos and order. Among the few ballers and fans that survived the basketball genocide was Charles Barkley, the man capable of performing the "Verboten Jam"..."
Shotgunnova has written an excellent detailed walkthrough for the game which is available here on GameFAQs (Local Mirror 12th Feb '08). It's also accompanied by two maps: Cuchulainn's Tomb and Spalding's Factory.
YouTube user Farky has videos of all the game's bosses and endings. The first boss video is shown here: