Thunder Spirits is a scrolling shooter video game for the Super Famicom/Super Nintendo released by Technosoft and Toshiba in 1991. The game is part of the Thunder Force series. It is a port of the dually copyrighted Technosoft/Sega arcade game Thunder Force AC, which itself is actually a modified port of the Sega Mega Drive/Genesis game Thunder Force III. The SNES version does away with level selection, changes several levels, and features a modified soundtrack, which takes advantage of the digitized instrument capabilities of the SPC700. Some of the graphics are considered improved compared to those two games, and some are worse. The Sega co-copyright could be why the game itself isn't called Thunder Force AC on the SNES.
A super-powered starship is under attack, and the pilot has to maneuver it over all sorts of terrain to make it out alive. Eight intergalactic stages make up the action in this fierce test of strength and agility. You'll have lots of powerful weapons to help you along the way, but firestorms and dangerous enemies should provide ample challenges to keep you on the edge of your seat. Just when you think you've made it through a level, you'll encounter gruesome boss characters that'll make you wish you were already dead. It's going to be one heck of a battle, but if you can buckle down and use your head, you might just live to tell about it. --Box Summary
There are eight levels in Thunder Spirits, each consisting of the ship battling numerous enemies while side-scrolling, usually with a "mini-boss" about halfway through the level. Then, at the end of each level, a boss (consisting of one or two enemies) appears, which must be defeated before preceding to the next level. Throughout the level, the ship can pick up "Weapon upgrades"; and the player is able to switch between any of up to five weapons at any time.
Source: Wikipedia, "Thunder Spirits", available under the CC-BY-SA License.