These comments assume viewers have watched previous 'Full Metal Panic' series.
After the events of 'Second Raid' Amalgam are far from defeated; under the leadership of Leonard Testarossa they soon fight back. Mithril's island base is overrun leaving survivors to flee aboard the submarine Tuatha de Danaan. Meanwhile Kaname Chidori is taken by Amalgam leaving protagonist Sousuke Sagara despondent. He travels to Namsac, a corrupt country. Here he gets involved in competitive mech-fighting. It isn't long before he is facing Amalgam again and working to rescue Kaname.
This is an exciting action packed series that almost entirely foregoes the comedy elements present in earlier series of FMP. The story is much more downbeat with things going badly for our heroes; there are some impressively emotional moments, these include a real sucker punch near the middle of the series. The action is impressively animated in a way that makes the viewer feel part of the action. The characters are good; this includes both established characters and those introduced in this series. The story progresses well but here we come to the series only real problem... the final episode felt more like the last episode before a mid-season break than a finale. I can only hope there will be another series to resolve certain very important plot threads. Overall though I'd certainly recommend this to FMP fans.
These comments are based on watching the series in Japanese with English subtitles.