Metroid Fusion sees Samus infected during a mission protecting scientists by a parasite called X. To save her life, the infected parts of her suit are surgically removed and she is injected with a vaccine made from a culture of metroid cells which also happen to be the X parasite's natural predator. The parasite has thrived since Samus herself has all but eradicated the metroid population. Now she is sent on her first mission since the incident to investigate an explosion at the lab where her infected suit was sent.
Developed by the same team behind the classic Super Metroid for SNES, Fusion is also of top quality. Controls are smoothed out and using the shoulder buttons for things like diagonal aiming and toggling between weapons makes combat much more fluid now that you don't have to stop and switch. Enemies are standard fare from other metroid games with the added caveat that they are actually the X mimicking the wildlife. Killing them turns the parasite into it's natural form which Samus can use to restore health and weapons since the metroid vaccine has immunized her to their parasitic nature. Samus has lost all of her abilities since the last game (As she always does), but they can be restored by downloading upgrades from HQ or absorbing large parasites that come from defeating bosses.
The game is less open than it's predecessors since there is actually a minor focus on story, but you can still just fuck off and explore for power-ups and hidden areas if you want. Some complained about adding back-story to Samus, but I think it was rather well done and humanized her a bit. You can also use the link cable accessory to connect with both Metroid Prime and Metroid: Zero Mission to unlock bonus content. All the classic Metroid you expect with smoother controls and pretty new graphics.
Buyer's Guide:Available used on the GBA for anywhere between 5 and 30 bucks depending on the seller. Also available on the Virtual Channel to 3DS owners.
4 She should learn to keep spare upgrades in a closet or something out of 5